The Magic Math Trick That Fools Everyone

What makes a simple brain-teaser about two identical swords one of the most deceptively tricky logic puzzles of the last century? And why has this seemingly innocent riddle fascinated mathematicians, programmers and puzzle-solvers for decades? First shared in recreational maths circles before spreading across classrooms and online forums, the “swords of truth” puzzle asks a disarmingly straightforward question — yet its solution reveals deep ideas about reasoning, information, and the limits of what we can deduce from what we’re told. Is the puzzle just a clever bit of misdirection, or a gateway into the foundations of logic itself? And why does a question about two swords feel so strangely relevant in an age of misinformation? Welcome to The Rest Is Science: Field Notes. Every Thursday, Hannah and Michael rummage through their personal troves of scientific treasure and source discoveries that explain our understanding the universe, oddities that scramble our brains, objects that hint at forces we’ll never see...and a few things that are essentially just plain cool. Expect deep dives into the science behind each pick, the spark that grabbed their attentions, and the sheer delight they get from sharing it all with you. They’ll also be tackling your questions, so email The Rest Is Science at [email protected]. ------------------- For more information about Cancer Research UK, their research, breakthroughs and how you can support them, visit ⁠⁠https://cancerresearchuk.org/restisscience⁠⁠ Cancer Research UK is a registered charity in England and Wales (1089464), Scotland (SC041666), the Isle of Man (1103) and Jersey (247). A company limited by guarantee. Registered company in England and Wales (4325234) and the Isle of Man (5713F). Registered address: 2 Redman Place, London, E20 1JQ. EXCLUSIVE NordVPN Deal ➼ https://nordvpn.com/restisscience Try it risk-free now with a 30-day money-back guarantee 🌍 ------------------- Find The Rest Is Science all over the internet by ⁠clicking here.⁠ ------------------- Video Producer: Adam Thornton Video & Social: Bex Tyrrell Assistant Producer: Imee Marriott Producer: Becki Hills Senior Producer: Lauren Armstrong-Carter Head Of Digital: Samuel Oakley Exec Producer: Neil Fearn Learn more about your ad choices. Visit podcastchoices.com/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Hannah Fry?

  This episode is brought to you by Cancer Research UK.

• 0:02Michael Stevens?

  Imagine this: inside all of us, billions of cells follow millions of instructions written in microscopic code. And when a new cell grows, it copies those instructions, but the smallest error can lead cancer to develop.

• 0:17Hannah Fry?

  Right, and this is the reason why there isn't a single cure for cancer, because, you know, there are more than 200 different types, each of them have got distinct characteristics, you know, different challenges, different mysteries. And that means that trying to cure cancer isn't like following a single path, it's like trying to map out an entire forest.

• 0:34Michael Stevens?

  That's right. And Cancer Research UK is the world's largest charitable funder of cancer research. I mean, their work spans more than 20 countries, with over 4,000 scientists, doctors and nurses pushing knowledge forward to save and improve lives worldwide.

• 0:50Hannah Fry?

  You know, over the last 50 years, the work that this charity has done has helped to double cancer survival in the UK. And you have to think about, that is more parents at the dinner table, right? That is more friends at their birthday parties, that is more people who are living longer, better lives.

• 1:08Michael Stevens?

  For more information about Cancer Research UK, their research, breakthroughs, and how you can support them, visit cancerresearchuk.org/rastescience.

• 1:19Speaker 02

  Observe: The North American Dad. 50s. Great polo collection. He enters his vehicle information on Carvana. An offer appears in minutes. Remarkable. He squints, reads it again, smiles and says, "Dang, that's a good offer." The highest form of approval. No haggling, no hassle, no one called him "boss." He's still on the couch. Astonishing. Sell your car today on carvana.com. Pickup fees may apply.

• 1:51Speaker 03

  This episode is brought to you by Google Chrome. You think you know a browser, but Gemini and Chrome, that's new. It can help you with practically anything on the web, like restoring a vintage motorcycle from a 50-page restoration block, or finally break down that long article you've had open for weeks. Gemini and Chrome is here for it. Ready to make anything online make sense? There's no place like Chrome. Check responses set up required. Compatibility and availability varies. 18+. Check responses set up required.

• 2:17Speaker 04

  Born from the intensity of a night in the eternal city. Introducing Born in Roma Fragrances by Valentino. Feminine perfume Donna Born in Roma features juicy black currant, central jasmine, soft cashmerin and warm vanilla. Masculine leaning but unisex Womo has notes of fresh violet leaf, aromatic sage, green vetiver and mineral salt. Shop holiday gifts at macys.com.

• 2:52Hannah Fry?

  Welcome to The Rest of Science. This is Field Notes. This is a kind of podcast expedition diary where Michael and I trade off on our exciting objects, our exciting discoveries, big questions that have been occupying our minds.

• 3:04Michael Stevens?

  Yeah, it's kind of like show and tell, but for nerds with a capital N.

• 3:10Hannah Fry?

  Absolutely. Every week, one of us is going to bring something strange, something quirky, something interesting, some object or some story. And yeah, we're going to chat about it together.

• 3:19Michael Stevens?

  That's right. And please send us your own weird stories, thought experiments or questions, because we will address them on this show.

• 3:28Hannah Fry?

  What have we got today?

• 3:29Michael Stevens?

  All right. It's basically a number of wooden swords on a keychain.

• 3:32Hannah Fry?

  Yeah.

• 3:32Michael Stevens?

  On a keyring. And each one has six two-digit numbers on it. And some of the numbers are surrounded by a square and some by a circle. And how many swords are there?

• 3:42Hannah Fry?

  Eight swords.

• 3:43Michael Stevens?

  Eight swords. And they're all what? Like, how many centimeters long?

• 3:47Hannah Fry?

  OK, so I would say that they're about 10 centimeters long. They have quite a sharp edge.

• 3:51Michael Stevens?

  Yeah, they do.

• 3:51Hannah Fry?

  That's one thing that's noticeable. They're sort of, they're engraved with these numbers along the side. Each of them has six numbers in total, two-digit numbers. And around each of these numbers is a shape, which is either a square or a circle.

• 4:08Michael Stevens?

  That's right. And they were made by Creative Craft House, a just wonderful group of people that make wooden puzzles and toys. I gave this to Hannah all the way back during our photo shoot. So we have talked about it. We know what it is. But today, we're going to tell you all about it.

• 4:23Hannah Fry?

  You know what, though? I think, I think people watching will be aware that our delight of nerdy things is absolutely authentic, that we do this in our own time, people.

• 4:31Michael Stevens?

  We did this in our own time. That's right. That's right. And we're just recreating the magic of that moment. So this one's a magic trick. So what I want you to do is look through the swords and pick one of the swords. And I actually cannot see what you're doing because of your laptop, so no trickery.

• 4:46Hannah Fry?

  Okay.

• 4:47Michael Stevens?

  Okay, now pick a number on that sword. Don't tell me what you've picked.

• 4:51Hannah Fry?

  I'm gonna pick this one here.

• 4:54Michael Stevens?

  Okay, you've got it. So now what I want you to do is to to read the numbers, well, not the numbers, but the shapes that surround them, and say the name of the shape, left to right. But when you reach the number you've picked, lie and say the opposite of what its shape is. So if it's a square, say circle. If it's a circle, say square.

• 5:11Hannah Fry?

  Okay, here we go. So we're gonna go circle, square, square, square, square, circle.

• 5:18Michael Stevens?

  Okay, circle, square, square, square, square, circle.

• 5:22Hannah Fry?

  Exactly.

• 5:23Michael Stevens?

  Okay, now what I need to do is I need you to think about your number. I want you to think about it very deeply.

• 5:32Michael Stevens?

  And then we're gonna speed this up because it's gonna take me a while to do this in my head because I'm out of practice, but I will come up with it. Hannah?

• 5:39Hannah Fry?

  Yes.

• 5:40Michael Stevens?

  Did you pick the number 36?

• 5:42Hannah Fry?

  I did pick the number 36.

• 5:44Michael Stevens?

  I could tell, I could feel it.

• 5:45Hannah Fry?

  Hang on, let me just check. Is there any other 36 on here? There is not. You you managed to get that exactly correct. How did you do that?

• 5:54Michael Stevens?

  Thank you for asking. I was able to deduce that you had chosen 36. Is deduce the right word? You told me. I knew you chose 36 because you told me.

• 6:05Hannah Fry?

  How did I tell you?

• 6:06Michael Stevens?

  In code, in binary.

• 6:09Hannah Fry?

  In binary code.

• 6:10Michael Stevens?

  So think of it this way. Imagine that the circles are actually zeros and the squares are actually ones. You gave me the digit 3 and the digit 6 because 011 is 3 in binary. But who wants to describe the rules of binary?

• 6:26Hannah Fry?

  I think you could do it.

• 6:27Michael Stevens?

  Okay, so we use a denary system, a decimal system, where our place values go from ones to tens to hundreds, right? A three-digit number is in the hundreds. And with binary, you're only going up by 2 each time. So you'll write down 0 for 0, 1 for 1, 2, nope, nope, we're not gonna do a 2. 2 moves us to another place value. We put a 1 and then a 0. So we have 1, 2, and no 1s. 1 0 is 2 in binary. It's 10 in hours.

• 6:58Hannah Fry?

  I mean, using base ten is for lame people who have to take their socks off when they want to count to twenty, basically. I'm not interested in that. I want something much more efficient. I want something where you can cram more in. And the way that you do that with binary, it sort of seems like when you start off, you know, your first value, it only goes up to one, you're zero or one, nothing else, right? It sort of feels like very inefficient. The second value, as I'm moving right to left, is two. It can be, if it's zero, it's zero. If it's one, it's actually a two.

• 7:28Michael Stevens?

  Right.

• 7:29Hannah Fry?

  Right? So, so one, one in binary is two plus one, which is three.

• 7:34Michael Stevens?

  That's right.

• 7:34Hannah Fry?

  Um, it feels like, hang on a second, you've got, you've got ages to go. Counting to base ten is so much more efficient. You can kind of cram way more in, but very, very quickly, binary numbers get you up to like extremely high levels. So, for example, on your hand, if you use this as binary digits instead, right? So, in our ordinary way of counting, you go one, two, three, four, five, six, seven, eight, nine, ten. You can count to ten on your

• 7:58Michael Stevens?

  On two hands.

• 7:59Hannah Fry?

  On two hands, right? I mean, lame.

• 8:01Michael Stevens?

  Yeah.

• 8:02Hannah Fry?

  Using binary, you can do way more. It's a way more efficient way of counting.

• 8:07Michael Stevens?

  And if I was doing base ten on my hand, then I could have the tens, hundreds, thousands, ten thousands, hundred thousands, just on my hand.

• 8:14Hannah Fry?

  You could, you could. One thing, very briefly, just, just to note, anytime someone flips you the double bird, right? In your mind, you can just think, this is a hundred and thirty-two.

• 8:24Michael Stevens?

  Hundred and thirty-two. Okay, so four is one of them.

• 8:27Hannah Fry?

  Yeah.

• 8:27Michael Stevens?

  And then two of them is

• 8:29Hannah Fry?

  Hundred and thirty-two.

• 8:29Michael Stevens?

  Hundred and thirty-two.

• 8:31Hannah Fry?

  So going back to the swords then, so actually if you just take the last three shapes, square, square, circle, is the same as saying like that.

• 8:39Michael Stevens?

  Which is six. It's one, four, and one, two combined. And as you can see, you can get any integer you want using this system. So, you told me, circle, square, square.

• 8:51Michael Stevens?

  Square, square, circle. Those first three, circle, square, square is three. It's a two and a one. Uh, and then square, square, circle, one, one, zero is six. So thirty six.

• 9:04Hannah Fry?

  The thing that I love about these, right? So I brought them home. I showed them to my children and um, who are eight and six, by the way. Oh, sorry, they are.

• 9:13Michael Stevens?

  Eight.

• 9:13Hannah Fry?

  Eight and six.

• 9:15Michael Stevens?

  Yeah.

• 9:16Hannah Fry?

  And they absolutely love them. I've managed to, using this, my guy, I've managed to trick them into learning binary numbers. Um, can't get them to do, you know, normal addition in their math homework, but binary numbers using the swords of truth.

• 9:28Michael Stevens?

  It's fun and it shows them that maths is not uh, like a whodunit, where we all know the answers already, that it is a practice. It's like a fun activity. It's a craft.

• 9:39Hannah Fry?

  Quite a lot of magic tricks actually have these mathematical elements to them, right?

• 9:44Michael Stevens?

  I know a lot of like ones that use decks of cards.

• 9:46Hannah Fry?

  Yeah.

• 9:48Michael Stevens?

  I mean, there's the old like one, oh, eight, nine trick. I don't know how it works though. But if you take a three digit number, whose first and last digits are different. And then you reverse it and you subtract the smaller from the larger. You take that number, you reverse it and you add both of those together. You will always get one thousand eighty nine.

• 10:11Hannah Fry?

  Okay.

• 10:12Michael Stevens?

  Yeah.

• 10:13Hannah Fry?

  So sometimes people use this to say, they get a book that's got more than a hundred pages. And they say, okay, you've got a four digit number. Go to the, what's your number? One, one, oh, eight, nine. Go to the hundred and eighth page. Go to the ninth word, long. Is it this thing here?

• 10:31Michael Stevens?

  Yeah. Yeah, you can create, you can embellish it to make the trick really mind blowing.

• 10:36Hannah Fry?

  Yeah.

• 10:38Michael Stevens?

  And those are my favorite kind of tricks because they work all the time. There's no sleight of hand and. I don't feel bad telling people how it worked because then I'm teaching math.

• 10:47Hannah Fry?

  Yeah. Hey, any opportunity to teach math, as far as I'm concerned, is absolutely fine by me.

• 10:52Michael Stevens?

  The Swords of Truth, the original like design of that trick was created by an inventor named Alex Elmsley. And he came up with a way to organize all these numbers and put squares or circles around them such that reversing any one of their shapes revealed that number's identity.

• 11:09Hannah Fry?

  Hmm.

• 11:10Michael Stevens?

  Which is brilliant.

• 11:10Hannah Fry?

  Yeah, it's a really cute trick. It's a really cute trick. When it comes to these Swords of Truth, though, I sort of had imagined that it would been, it was an error correcting code.

• 11:18Michael Stevens?

  What's that?

• 11:19Hannah Fry?

  If you've come across these things. They are a very clever little maths trick that's not used in magic, but instead is used across the board to make basically the modern world function. So if you imagine that you've got a Sudoku, okay, and I sort of pass you Sudoku, but like a bit of the corner is rubbed out. You don't need to see the original Sudoku to know what those numbers were. It's possible for you to work it out. So now, when you have a barcode, for instance, on an item, and the item gets like bashed around the supermarket, it gets to the checkout, you don't need to have a perfect reading of the entire barcode in order to be able to fill out and work out what all the numbers are.

• 12:03Hannah Fry?

  Because what it has is things called parity checks in it, so that it just needs a certain number of the numbers. And then there'll be extra rules in there, like all of these numbers must add up to a hundred or every alternate digit will be even, so that you can still work out what it is, even if you don't have the entire barcode itself.

• 12:24Michael Stevens?

  Okay, so you thought maybe there's a rule like, oh, there's always a pattern to the circles and squares? And how it deviates tells you what the number is.

• 12:32Hannah Fry?

  Yeah, that you're lying.

• 12:32Michael Stevens?

  That's actually what I thought first, too.

• 12:34Hannah Fry?

  Yeah, absolutely. You know that thing about about error correcting codes, by the way? So QR codes are an example of error correcting codes. You actually can cover up a third of a QR code and it'll still work absolutely fine. Which I think, for starters, gets sort of gamed by companies who put their logo in the middle of the QR, you see this, right? And they do this, not because you don't need that bit of the QR code. They're just using up the fact that you can cover up thirty percent of it.

• 13:02Michael Stevens?

  So they've lost some of the error correcting ability.

• 13:04Hannah Fry?

  They've lost some of the error correcting ability. You cannot cover up a third of one which has a company logo in the middle.

• 13:11Michael Stevens?

  Right, right. It would have to be a third when you combined with the logo.

• 13:16Hannah Fry?

  Exactly. But I think quite a lot of people often get caught out by this. So the classic one is where people post photos of their boarding passes when they're like going on a trip.

• 13:25Michael Stevens?

  Ah.

• 13:26Hannah Fry?

  Because even when you cover it up with your thumb or whatever, because you can recover a QR code, even when you don't have the complete QR code itself. So there have been instances of people posting their boarding cards being like, oh, look, I'm flying to this place tomorrow. And then other people on the internet managing to go through, logging on to the airline, changing the seat for the individual, maybe getting them to sit next to the toilet. You know, maybe cancelling their flight, changing their, changing their flight dates.

• 13:55Michael Stevens?

  Putting them in the middle seat.

• 13:56Hannah Fry?

  Putting them in the middle seat, exactly. There was one particularly viral case where someone changed a stranger's seat to have them sitting next to the toilet and posted online saying, you showed enough pixels to resurrect Jesus, let alone a QR code.

• 14:13Michael Stevens?

  Well, look, after the break, we're going to resurrect Jesus.

• 14:18Michael Stevens?

  Or will we? Probably not, but who knows what we're going to do? We'll see after the break.

• 14:29Michael Stevens?

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• 15:06Michael Stevens?

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• 15:23Hannah Fry?

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• 15:40Speaker 03

  This episode is brought to you by Google Chrome. You think you know a browser, but Gemini and Chrome, that's new. It can help you with practically anything on the web, like restoring a vintage motorcycle from a 50-page restoration block, or finally break down that long article you've had open for weeks. Gemini and Chrome is here for it. Ready to make anything online make sense? There's no place like Chrome. Check responses set up required compatibility and availability varies 18 plus.

• 16:09Speaker 04

  Born from the intensity of a night in the Eternal City, introducing Born in Roma Fragrances by Valentino. Feminine perfume Donna Born in Roma features juicy black currant, central jasmine, soft cashmerin, and warm vanilla. Masculine leaning but unisex Womo has notes of fresh violet leaf, aromatic sage, green vetiver, and mineral salt. Shop holiday gifts at macys.com.

• 16:38Hannah Fry?

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• 17:14Michael Stevens?

  All right, we're back. I'm gonna right a wrong.

• 17:17Hannah Fry?

  Okay.

• 17:17Michael Stevens?

  Because I found out that I do not follow you on YouTube. And I noticed like three weeks ago and I'm like, I'm gonna wait for a Field Notes episode to do it and so everyone can watch. Here I am, I've opened up YouTube. Here I am, here's my little like channel page. I'm gonna search for Fry Squared.

• 17:34Hannah Fry?

  Fry R squared. It's a maths joke, Michael. Come on.

• 17:37Michael Stevens?

  Oh, it's Fry R squared. I don't even know the name of your YouTube channel. Fry R squared is so much better than Fry squared.

• 17:44Hannah Fry?

  Isn't it?

• 17:46Michael Stevens?

  Holy cow.

• 17:47Hannah Fry?

  I was choosing between that and E to the I Fry.

• 17:49Michael Stevens?

  That's really clever. Oh my gosh. All right, now here's the proof. Here I am, logged in as Vsauce and the subscribe button is ready to be clicked. We're gonna watch it all together.

• 17:58Hannah Fry?

  Yeah.

• 17:59Michael Stevens?

  Boom.

• 18:00Hannah Fry?

  Is this the point that I should admit that yesterday I realized I also didn't subscribe to Vsauce?

• 18:06Michael Stevens?

  Oh, and do you now?

• 18:07Hannah Fry?

  And secretly. Just quickly press the button. I wanna see how many people you follow. Hold on, let's have a look.

• 18:14Michael Stevens?

  Oh, there's one. I don't know who it was, but someone I followed years ago, they just changed their name. Or someone bought the channel and it's now called like Totally Nude Massage? Something like that. And there's this whole debate about like, why is Michael subscribed to this channel? It's because that wasn't what it was originally called.

• 18:34Hannah Fry?

  Hey, look, I think lean into it.

• 18:36Michael Stevens?

  I wanna own it. I wanna be like, yeah, you guys keep wondering why. Cause if I unsubscribe, it's like I was embarrassed or they caught me at something.

• 18:46Hannah Fry?

  This is true. This is true. I think just, just keep teasing it and maybe in 2035, release a video on Totally Nude Massage.

• 18:54Michael Stevens?

  Yeah, a colla- a little collaboration.

• 18:58Hannah Fry?

  Hey, why not?

• 18:58Michael Stevens?

  Why not? You know, I was thinking during the break that people can be resurrected, but can someone be zerected?

• 19:08Michael Stevens?

  Because to resurrect means to be resurrected. Like, oh, man, I was resurrected again.

• 19:15Hannah Fry?

  It's a bit like, the one that people always talk about is whelm, right? Where it's like you can be underwhelmed or overwhelmed, but neverwhelmed.

• 19:21Michael Stevens?

  You can be under the weather, but you can't be over the weather.

• 19:24Hannah Fry?

  No.

• 19:25Michael Stevens?

  Can you be the weather? I'm gonna try later today after the recording.

• 19:31Hannah Fry?

  Be the weather?

• 19:32Michael Stevens?

  I'm full of questions, but I want to hear questions from our viewers.

• 19:35Hannah Fry?

  Yeah, okay, so we have a little mailbag, little mailbag entry here. Lucy has asked, if you couldn't live on Earth, which planet would you like to live on and why? Or Moon, I'm gonna expand it to Moon.

• 19:48Michael Stevens?

  Fair enough.

• 19:50Hannah Fry?

  There are some quite fun, fun ideas. I mean, one of them's got some rain's diamonds, doesn't it?

• 19:55Michael Stevens?

  Yeah, but I'm gonna say Venus, but I would wanna live on like a blimp up in its atmosphere where the pressure, the atmospheric pressure is safe. And gravity would be like pretty similar to Earth's. They're like twin planets, Venus and Earth.

• 20:12Hannah Fry?

  Yeah.

• 20:12Michael Stevens?

  I couldn't handle having less gravity on Mars. I think that would be, even though Mars is a fun one, even if we terraformed it, made its atmosphere breathable, it would still just not have enough gravity for the muscle strength I wanna build.

• 20:26Hannah Fry?

  Hmm, to be hench.

• 20:29Michael Stevens?

  Just to, yeah, just to, I don't know, I'm worried about that. I feel like even my lungs wouldn't be strong enough. And I, I feel like if I stayed on Mars too long, I would never be able to come back to Earth.

• 20:39Hannah Fry?

  Yeah, what about the other way around? If you went somewhere with more gravity and then just did loads of weight training, you could come back to Earth and be like extremely buff.

• 20:50Michael Stevens?

  Yeah, I mean, there's a limit to how strong a human could get, but yeah, I've heard from people who've done the vomit rocket thing. Have you done that?

• 20:57Hannah Fry?

  No, never.

• 20:58Michael Stevens?

  Okay, the zero G flights where you go up in a plane and then the plane just drops and you're in free fall inside the fuselage of the plane. So it's like you're in, in a free fall orbit around the Earth. Your legs are one of the biggest hindrances you have.

• 21:12Hannah Fry?

  Really?

• 21:12Michael Stevens?

  Because people have a tendency to go, oh, I wanna move and they'll kick themselves to move like they're swimming, but there isn't the resistance of water around them and they fly across the room. And if you watch videos of people doing this, you'll notice that they can't control how powerful their legs are. So if you could go and live on, I mean Jupiter's too much, but you know, in the atmosphere of Jupiter, far enough away that the gravity was twice or three times Earth's. Came back to Earth, you would be like demolishing your shoes. You would, the steps would just crumble at your feet.

• 21:45Hannah Fry?

  It's a way, I really want to understand that, that comet rocket thing. So wait, when they're pushing off from something, not just like kicking in the air, right?

• 21:52Michael Stevens?

  Yeah, kicking in the air does nothing.

• 21:54Hannah Fry?

  But if they're pushing off from something, they're suddenly extremely strong.

• 21:57Michael Stevens?

  Yeah, they'll be like, oh, I want to move away from the wall. And they just push themselves as you might if you were in a rolly chair or if you were underwater. But that winds up being 20, 30 times the force they need. And they just, and also the force doesn't die down, right? They just keep moving. And so if you kind of tap yourself to move into the middle of the plane, nope, you're gonna hit the wall.

• 22:17Hannah Fry?

  Okay, but the reverse of that then is that if you did go and live on Mars, you would feel like a kangaroo?

• 22:22Michael Stevens?

  Yes, you would.

• 22:24Hannah Fry?

  That would be pretty cool.

• 22:24Michael Stevens?

  That would be very fun. I think it's more of a theme park ride than a home, and I'm looking for a home.

• 22:29Hannah Fry?

  Mm hmm. You're gonna actually live there. I do always think when people talk about, you know, terraforming Mars and it's like, oh, all we've got to do is, is add in some atmosphere and then da, da, da, and then da, da, da. It's like, you know, guys, there are, there are inhospitable places on Earth that are way more hospitable than that. Like, why don't we just go and, I don't know, the Sahara, for example? Well, it needs a little bit of water and you're fine.

• 22:53Michael Stevens?

  But it's got, it already checks off a lot of boxes. It's got the right amount of gravity, it's got the right kind of air.

• 22:57Hannah Fry?

  Yeah.

• 22:58Michael Stevens?

  I guess a place where the wet bulb thermometer temperature gets up to a certain point, uh, that would be a fun challenge.

• 23:06Hannah Fry?

  Hmm.

• 23:07Michael Stevens?

  You know, the wet bulb temperature?

• 23:08Hannah Fry?

  Go on.

• 23:09Michael Stevens?

  So this is, this is not the air temperature that a thermometer reads. It's the temperature as measured by a thermometer that's wrapped in a wet towel. So it shows you what evaporative cooling is able to do.

• 23:20Hannah Fry?

  Oh.

• 23:21Michael Stevens?

  And if the wet bulb temperature gets above 95 Fahrenheit or 35 Celsius.

• 23:26Hannah Fry?

  That's not that high.

• 23:27Michael Stevens?

  That, that's too high for your body to cool down and you will die.

• 23:32Hannah Fry?

  Hold on, hold on, hold on, hold on. Right, if you wrap a wet towel around a thermometer and then take it outside on, I mean, we can still do this on Earth.

• 23:44Michael Stevens?

  Yeah.

• 23:44Hannah Fry?

  If that goes above 35 degrees. You will die.

• 23:48Michael Stevens?

  Yeah.

• 23:48Hannah Fry?

  Because your body will not be able to maintain body temperature.

• 23:51Michael Stevens?

  Evaporative cooling cannot save you.

• 23:53Hannah Fry?

  Which of course is how you how you cool yourself down, right?

• 23:55Michael Stevens?

  Which is how you cool yourself down. And so if you have cold water, if you've got refrigeration, if you've got an air conditioner, you will be okay. But we are finding more and more places on earth that are exceeding that wet bulb temperature, sometimes just for a day. But the longer you stay in that zone, the more people die. They have to escape. They cannot keep their bodies at a safe temperature at that level.

• 24:19Hannah Fry?

  Wow.

• 24:20Michael Stevens?

  Yeah.

• 24:21Hannah Fry?

  That's extraordinary.

• 24:22Michael Stevens?

  Yeah.

• 24:23Hannah Fry?

  What's the hottest place you've ever been?

• 24:24Michael Stevens?

  Oh, the hottest place I've ever been is Death Valley, which, by the way, it keeps setting records for highest temperature dry bulb. And I think if you went out there with enough like equipment, you could detect a record high number in August.

• 24:38Hannah Fry?

  You could hold the record, is that what you're saying?

• 24:41Michael Stevens?

  Yeah, right, right, right, right. So I've always wanted to do that as a YouTube video. I'm like, just in August, like, get a couple of weeks, stay out in Death Valley. And you can record the hottest temperature ever recorded. They're happening out there all the time.

• 24:52Hannah Fry?

  This does feel like a quite a high risk way to get a get a record, Michael.

• 24:56Michael Stevens?

  Uh, it's worth it, though.

• 24:59Hannah Fry?

  Died doing what he loves.

• 25:01Michael Stevens?

  I wouldn't, I wouldn't die, but if I did, think about how many views the video would get.

• 25:08Hannah Fry?

  It's all content, baby.

• 25:09Michael Stevens?

  Yeah. Yeah.

• 25:11Hannah Fry?

  Tell you where I wouldn't want to live in the solar system. I think Triton is the is the absolute worst place imaginable.

• 25:18Michael Stevens?

  Triton is a moon of Neptune. That's far away.

• 25:22Hannah Fry?

  Mm hmm. Super cold. It's got geezers that run on liquid nitrogen. You know, spouting out minus two hundred and thirty fountains into this into the sky. Also, it orbits backwards.

• 25:35Michael Stevens?

  Oh.

• 25:36Hannah Fry?

  Yeah, just to really mess with you.

• 25:40Michael Stevens?

  I think Triton would be beautiful, though. Doesn't it possibly have oceans of like methane or something?

• 25:45Hannah Fry?

  Yeah, I think so. I think so.

• 25:47Michael Stevens?

  That could be cool.

• 25:48Hannah Fry?

  Oh, no, wait, isn't methane on Titan? That's the one with the methane lakes. I think there the atmosphere is so dense that you could potentially fly.

• 25:57Michael Stevens?

  Yes, yes, I've heard this.

• 25:59Hannah Fry?

  Flapping your arms.

• 26:00Michael Stevens?

  Because the atmosphere is very thin, but the gravity is so low that human arms, they've calculated, could flap and you could fly.

• 26:09Hannah Fry?

  That would be pretty cool.

• 26:11Michael Stevens?

  That would be very cool.

• 26:12Hannah Fry?

  I mean, it would stink.

• 26:14Michael Stevens?

  Okay, you're not gonna take a helmet off, I think. You're just gonna wear your spacesuit. And, like, yeah, on the moon, gravity is very low, but there's no atmosphere. So moving your arms doesn't displace enough mass to move your mass. But on Triton, that's where you can.

• 26:29Hannah Fry?

  Titan, I think.

• 26:29Michael Stevens?

  On Titan.

• 26:31Hannah Fry?

  I mean, look, they all end up being the same, don't they, really? Titan, you've.

• 26:34Michael Stevens?

  Titan, Triton, the moon. How funny that the moon is just called the moon.

• 26:41Hannah Fry?

  Yeah.

• 26:42Michael Stevens?

  It just, it doesn't have a, does it have a name in any other languages?

• 26:46Hannah Fry?

  I don't know, you know?

• 26:47Michael Stevens?

  There, there must be cultures that are like, oh, no, it doesn't have the general name for a satellite around a planet. It's called, you know, Jeremy, or whatever.

• 26:58Hannah Fry?

  Maybe the Martians called it something different.

• 27:00Michael Stevens?

  Oh, yeah.

• 27:01Hannah Fry?

  You know that thing about life on Mars, right?

• 27:03Michael Stevens?

  Which thing?

• 27:04Hannah Fry?

  I mean, just the, the idea of whether.

• 27:06Michael Stevens?

  Oh, whether it's there or was there, yeah.

• 27:08Hannah Fry?

  I got to go and see Curiosity before it launched.

• 27:14Michael Stevens?

  Oh, you did?

• 27:14Hannah Fry?

  I did. It was amazing.

• 27:16Michael Stevens?

  That is so cool.

• 27:17Hannah Fry?

  I wasn't allowed in the vacuum.

• 27:18Michael Stevens?

  So you weren't able to touch it?

• 27:20Hannah Fry?

  I wasn't able to touch it. I mean, they're extremely careful about who touches it. But I got to go and see the mockups of it in the Mars, basically, robot playground, where they kind of move it around and, and learn how to, to navigate. Couple of things that were really amazing about it. When they're checking for life on Mars, the idea is that they're, what Curiosity is gonna do is it's gonna drill into the rock and then take these samples that can be analyzed for signs of, of previous microbial life. And in particular, they chose a spot on Mars, which they think is a really old riverbed, where they, they think there used to be water flowing. And the idea behind that is that then when they're sampling in that space, they're not just sampling, oh, here's a patch of land.

• 28:05Hannah Fry?

  They're sampling something which has had land come in from all across the region.

• 28:09Michael Stevens?

  Great, yeah.

• 28:10Hannah Fry?

  Which is just really, really smart, right? But the thing that is, I found so amazing about Curiosity in, in this, like, process to, to analyze whether there's life on Mars, is as they're drilling in the rock, they're not like. Running the analysis on the samples, they've got another mission planned to go and get those samples and then another one to get them into orbit and then another one to come back to Earth to sort of analyze them. So at the moment, what Curiosity does is it drills into the rock, kind of puts this, these samples in these like basically test tubes and then chucks them on the ground. Just litters the surface of Mars with these like scientific samples.

• 28:47Michael Stevens?

  Waiting for us to do a mission to pick them up.

• 28:50Hannah Fry?

  Yeah. And then it's like, well, have you got, do you know how you're gonna do that? And then I, no, no, no idea. Yeah. You know, sort of many billions of pounds later.

• 28:59Michael Stevens?

  That's so cool. So, so yeah, cause I hadn't heard any results from it, but it's because we, Curiosity didn't analyze the samples.

• 29:06Hannah Fry?

  No, just chucked them on the ground, chucked them on the ground.

• 29:09Michael Stevens?

  Yeah.

• 29:09Hannah Fry?

  The other thing is, they're very careful about who gets to go in and touch the rover, of course.

• 29:13Michael Stevens?

  Yeah.

• 29:14Hannah Fry?

  Um, but they're also extremely careful about the engineers who do get to work on it, not leaving their mark on it. Because I think in the early days, they found people had like, you know, maybe with a sharpie, just like written their child's name on it, whatever. And they, they have this like blanket rule, you're not allowed to do that. But then they found that some people had started soldering or soldering, if you're in the American.

• 29:37Michael Stevens?

  Soldering, thank you. We like our silent L's, isn't that cool?

• 29:42Hannah Fry?

  Blows my mind that you call it soldering. Anyway, had like soldered in to the electrical circuits in this like microscopic way, the names of their children. But they're really like, no, this is from all of Earth.

• 29:53Michael Stevens?

  Yep.

• 29:54Hannah Fry?

  This is not coming from you as the engineer.

• 29:56Michael Stevens?

  I like that. I like that. I mean, obviously you want to avoid contamination of biological agents because you touch the rover and some virus stays on there dormant until it gets to Mars. And then there could be life there, very simple life, completely unprepared for that virus and you've literally extincted the whole planet.

• 30:17Hannah Fry?

  Hmm.

• 30:18Michael Stevens?

  But then to not even put a little message, your child's name on the rover. I like, I like that because right, the anonymity of the individual, but the power of the species. This is from humanity. But we'll put an American flag on it. But it's from humanity. One small step for a man, one giant leap for mankind. American flag.

• 30:43Michael Stevens?

  There is no Earth flag, though. And there should be, and there will be, I think, soon.

• 30:47Hannah Fry?

  Yeah. And once there is, then maybe the moon will get a proper name.

• 30:52Michael Stevens?

  Mooney.

• 30:53Hannah Fry?

  Mooney McMoonface.

• 30:54Michael Stevens?

  Mooney McMoonface, if the Brits have a say. The Americans would probably call it something like...

• 31:00Hannah Fry?

  Freedom Rock.

• 31:02Michael Stevens?

  The 51st state.

• 31:04Hannah Fry?

  Amazing.

• 31:05Michael Stevens?

  Would be pretty American.

• 31:06Hannah Fry?

  That would be pretty American. I think they would call it something like the Great American Skyball.

• 31:13Michael Stevens?

  Skyball. New Texas.

• 31:23Michael Stevens?

  This episode is brought to you by Cancer Research UK.

• 31:26Hannah Fry?

  You know, when the moon landing happened, there was all of this spin-off technology that made everybody's lives better, right? Nappies, cat scanners, memory phone. One big grand challenge that sparked all kinds of scientific innovation.

• 31:39Michael Stevens?

  I do, and things like that happen, not just in space exploration, but even in cancer research. Like, I don't know what the analogy to a moon landing in cancer research would be, but the coolest thing I've learned recently is that the old idea that, well, cancer is what happens when there's damage to your DNA just hasn't really turned out to always be the case. The Cancer Grand Challenges team, Mutographs, they looked at 7,800 cancer samples, and they found that there's this thing that they're calling dark carcinogens.

• 32:09Hannah Fry?

  Go on.

• 32:09Michael Stevens?

  Okay, these are molecules that cause cancer without leaving any scar on DNA at all.

• 32:15Hannah Fry?

  Right. I mean, that's completely against the previous theories, though.

• 32:18Michael Stevens?

  Yeah, so the new hypothesis, or one new hypothesis, is the promoter hypothesis, that there are other kinds of molecules that promote the expansion of mutated cells. And without those promoter molecules, mutated cells would just, like, stay dormant.

• 32:35Hannah Fry?

  Okay, so the mutated cells are still there, but it's like they need a little friend to help them.

• 32:39Michael Stevens?

  That's right.

• 32:39Hannah Fry?

  Just an evil friend who causes cancer.

• 32:42Michael Stevens?

  Exactly. It's not just let's pin this on DNA damage.

• 32:46Hannah Fry?

  Right.

• 32:46Michael Stevens?

  Kind of like you can't pin a mudslide on rain.

• 32:49Hannah Fry?

  Mm.

• 32:49Michael Stevens?

  There's also other factors like deforestation and there's other causes that all combine together. So it's more than just damage to DNA. Because, for example, you would think, okay, well, if this damage can like randomly happen, then an animal with more cells should get more cancer.

• 33:07Hannah Fry?

  Like elephants, for example.

• 33:09Michael Stevens?

  Exactly. Elephants, giant whales, right? They've got so many more cells than us. If each cell is like a lottery ticket and some of them unfortunately win the cancer, well, then we should see more cancer in bigger animals, but we don't.

• 33:22Hannah Fry?

  Does that also help to explain then why you have these patterns in the way that cancer appears across human populations around the world that can't totally be explained by mutations alone?

• 33:36Michael Stevens?

  That's right. It does. And it also leads us perhaps towards better ways of preventing cancer in the first place.

• 33:42Hannah Fry?

  Because you can target the promoters, not just the cells themselves.

• 33:45Michael Stevens?

  That's right. And so, you know, there's been a lot of follow up on this. One team, Prominent, is now chasing down what these newly identified promoters could be.

• 33:55Hannah Fry?

  This is like a clear demonstration that even if you haven't yet cured cancer, just doing the research is giving you all of this extra, these extra kind of hooks to latch on to. The more you know about it, the more you know about how it arises, the more chances you have of being able to develop a weapon that helps defend against it.

• 34:14Michael Stevens?

  That's exactly right. And for more information about Cancer Research UK, their research, breakthroughs, and how you can support them, visit cancerresearchuk.org/restiscience.

• 34:32Michael Stevens?

  All right, here's a question from James. He asked, I'm curious to know how we determine the age of water on Earth?

• 34:39Hannah Fry?

  Hmm, because we mentioned this in our water episode, right? We said that it was older than the sun, and then we gave no receipts.

• 34:45Michael Stevens?

  Yeah, how do you know?

• 34:47Hannah Fry?

  Okay, so there's a few different ways that you can tell. It works in a similar way to carbon dating effectively, which is that different atoms have different numbers of neutrons in them, right? So inside an atom, in the nucleus of an atom, you have the number of protons and that tells you what type of atom it is. And in hydrogen, for example, you can just have one proton on its own, or you can have a proton and a neutron, or you can even have a proton and two neutrons.

• 35:15Michael Stevens?

  Deuterium.

• 35:16Hannah Fry?

  Absolutely, like two with a blocked nose.

• 35:19Michael Stevens?

  Wait, two neutrons would be tritium, wouldn't it?

• 35:22Hannah Fry?

  Correct, yes.

• 35:23Michael Stevens?

  All right.

• 35:24Hannah Fry?

  So you have one on its own, hydrogen atom, which is just a proton.

• 35:27Michael Stevens?

  Just normal hydrogen.

• 35:29Hannah Fry?

  If you have something else stuck to it, a neutron, deuterium.

• 35:33Michael Stevens?

  Deuterium, two particles in the nucleus.

• 35:35Hannah Fry?

  Two tritium.

• 35:36Michael Stevens?

  But it's still hydrogen.

• 35:37Hannah Fry?

  Still hydrogen, right? Add another one in, tritium.

• 35:41Michael Stevens?

  Still chemically hydrogen, but it's called tritium.

• 35:43Hannah Fry?

  Absolutely. Now, the thing is, is that over time, this hydrogen atom sort of doesn't like the neutrons sticking around too much, so over time it will like kick them out, sort of be like, get out of here, I'm not interested in you. And we know the rate at which it kicks them out.

• 35:59Michael Stevens?

  And it's slow.

• 36:00Hannah Fry?

  It's slow, like millions of years slow.

• 36:03Michael Stevens?

  Right, and there are a lot of hydrogen atoms in a tiny bit of water.

• 36:07Hannah Fry?

  Absolutely. So the thing is, is that when water is formed, there's like a particular fraction of them that have deuterium, tritium, and then you can take a chunk of water and go in to look at how many of them of each of, you know, normal hydrogen, deuterium, and so on. And that can give you a good clue as to how old the water is. And there are a few other versions of this, right? So oxygen also has that same trick, right? You get heavier oxygen with more neutrons in it. And what you can do is you can, you can tell when that oxygen was formed as part of the hydrogen bond.

• 36:48Hannah Fry?

  And there's particular ratios that sort of match the ratio that you found in the earliest Earth, the sort of, this mixture of molten rock and then, and then the big, was it Theia that came in and sort of?

• 37:01Michael Stevens?

  Yeah, that's the name for the proposed, I think it's quite popular as a theory, the name of the thing that crashed into Earth to create the moon, Theia. It's probably about the size of Mars. Yeah.

• 37:13Hannah Fry?

  And the moon is just a little offshoot, a little splash.

• 37:16Michael Stevens?

  Yeah, it's made out of the guts of Earth and I guess Thea too.

• 37:19Hannah Fry?

  Absolutely. So we know that some of the water on Earth predates our moon because we know that from the oxygen isotopes. Then there's also the ancient water has dissolved noble gases in it. So things like xenon, argon, that kind of thing. And when that water has been locked inside sort of minerals in the mantle, so locked inside the rock, kind of underneath the surface of Earth. When you analyze that, you can see that there are xenon in there, for example, which has isotopes that you can tell how old it is from that. And then finally, zircon crystals, which are by quite a long stretch, my favorite gemstone.

• 38:03Hannah Fry?

  Zircon forms effectively like a cage, right? As it solidifies into crystal structure and can trap water in there. And so we know there's very, very, very old zircon crystals that have been found in Australia that tell us how old the Earth is, 4.3 billion years old. And we know that that rock could only have been formed in the presence of liquid water.

• 38:28Michael Stevens?

  So James, there you go. If anyone else out there has questions, please send them to us. You can email us at [email protected].

• 38:37Hannah Fry?

  Or you can join our newsletter. And we're going to be back every Thursday with a new edition of Field Notes and on Tuesday with our normal episode.

• 38:43Michael Stevens?

  Yeah, that's right. So we will see you then and there.

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