Skip to main content

XKCD’s Randall Munroe on his new book How To and the joys of using science to build lava moats

XKCD’s Randall Munroe on his new book How To and the joys of using science to build lava moats


Maybe don’t try the lava moat thing at home, though

Share this story

Photo by Amelia Holowaty Krales / The Verge

XKCD author Randall Munroe is no stranger to answering strange science questions, but his latest book, How To: Absurd Scientific Advice for Common Real-World Problems, might be his most practical science guide yet.

How To, which hits stores today, is a quasi-sequel to Munroe’s book What If?: Serious Scientific Answers to Absurd Hypothetical Questions, which was based on a series of blog posts Munroe wrote as a spinoff of his webcomic.

How To aims to be a more practical guide by using math and science — taken to the absolute extreme, in Munroe’s typical style — to answer basic questions about life, like how to charge a cellphone, how to take a selfie, or how to mail a package. While you might not be building a machine gun-powered jetpack (like Munroe explores in What If?), odds are you do power your home and send files, which gives How To’s absurd explorations a relatable grounding in day-to-day tasks.

The Verge recently spoke with Munroe about How To, the questions that didn’t make the cut, and the joys of building a lava moat (with science).

This interview has been edited for clarity and brevity.

Is there a natural evolution in your mind from What If? to How To? Is this more of a straight sequel with different questions?

Well, it’s partly that in What If? I was answering questions that other people sent in, and then I was trying to connect it up to interesting research I had read, or I did some calculation I wanted to share. And with this, I started thinking about household tasks I might want to do or problems I might want to solve, and just trying to think of “What are all the different ways you could solve this, and what would the consequences be?” And in some cases, I stumbled on some interesting piece of research and then wanted to talk about that. There’s a little infographic about a seismology institute that accidentally picked up a Bruce Springsteen concert on their seismograph from people dancing. And based on the beat of the dancing, they could figure out which songs were playing when in the set. And that was cool, so I did a little thing about how to listen to music. But in other cases, it was just me taking a problem that I’m always struggling with, like how to keep my phone charged, and then just trying to think of all the interesting ways I might solve it.

The move here seems to be to pull the thread and then take it to the absolute furthest extreme that it can physically go within the natural laws of our Universe.

Yeah, I think that’s sort of my MO, in general.

Is there something that appeals to you about applying math and science like this?

I think what appeals to me is that it’s cool that math and science let you get answers when you do that. I feel like everyone kind of looks around and wonders, “Oh, I saw a giant ant in this movie. I wonder how big ants can get?” Or “If I were in space, and I’ve wanted to deliver a package to the ground, could I just throw it out the window? How does that work?” And what’s really fun is that some of these problems, you don’t even know where to start solving that because there’s no easy way to try it. But you can actually get an answer through some straightforward calculations. It’s sort of like magic that you can just write down these spells on paper and get something that might completely surprise you. And it tells you something about the world. 

“You can just write down these spells on paper and get something that might completely surprise you.”

I had a friend who texted me that he had ants in his house, and he wanted to know, “Could I build a lava moat around my house? Or would it be prohibitively expensive to keep it heated?” And I had no idea of what even the order of magnitude of the cost of a lava moat would be. But I was just on my phone, and I was like, “I know they do thermal mapping from above, and I know most of the heat is lost to the air. I bet I can find a paper on this.” So I dug up this paper on heat flux from lava flows, and it gives it in watts per square meter. If you figure out how many square meters this moat is going to be, then simple multiplication will tell you how many watts of heat are flowing off of this, and that’s how many watts you need to supply it at minimum to keep it molten. Then you just multiply that by the price of electricity in my friend’s town, and bam! You have an actual number of dollars. It just seems very cool to get like real answers like that. “It would cost $40,000 a day to keep this moat molten,” which is not something I think you can figure out any other way.

When writing How To, did you come up with a question and then find a paper that helped solve it? Or did you find interesting or weird research and then come up with a question that it could help answer?

I think it’s a pretty even mix of both. Maybe more for How To, it’s more ones where I had the question first. But I feel like there’s a lot of both. You know, there was the Bruce Springsteen concert that I mentioned. I was reading legal studies on how property law works (when the ground moves) because I was sort of wondering how you deal with that. After I learned about it, I was like, “Oh, well, is there is there a thing that I can work this into?” But when I read a paper or some piece of research, it’s not so much that I think, “Okay, what’s the how-to guide that I could apply this to?” It’s more like, “Hey, would it be possible to use this to accomplish X?” Then, if the answer is “maybe” or “it’s surprisingly possible” or “no, but for an interesting reason,” then I say, “Okay, well, I’m going to write a guide ‘How to do X.’”

In the case of the property law paper, the laws are really ambiguous about this. So it suggested that you could do some kind of incredibly impractical heist where you buy property, but your neighbor has much more valuable property. But because of ground movement, you could then go to court and say, “Well, this three inches of your property has actually moved into mine, and so I get to take possession of that.” And gradually, you’d gain your neighbor’s property while your other neighbor gains from you on the other side.

It’s the world’s most impractical heist. But I like that, in our legal system, you could kind of do it. I just think that’s endlessly entertaining.

Were there any questions that were just too much to include, where either math didn’t work out or you couldn’t find corroborating research to support it?

There’s one, in particular. One of the things that I kind of had to figure out early on was, “How much do I want to write guides to questions that people really want to know how to solve?” Because it can be sort of frustrating if you’re reading a guide that’s “How to do this thing,” and it’s something that you really don’t know how to do but would like to know how to do. It feels like there is a good answer out there that research might be able to find. In that case, I feel like it could be sort of frustrating to write a thing where it’s like, “Let’s explore these fantastical solutions” when you want to know the real solution. 

So I originally thought it would be fun to have an article on how to dry out your phone if it falls in the water. And there’s the sort of “bag of rice” question, and everyone thinks, “Do you put it in a bag of rice? Does that really help? If it falls in saltwater, should you rinse it with fresh water?” That’s sort of counterintuitive, but it seems very plausible that that might help. And then I looked at impractical things like, “Well, could you just evaporate the body of water that it fell into?” Like, if you don’t want to get down to the bottom of the lake and get yourself all wet, could you dry out the lake and then just pick it up? And then how much can you heat up the phone before the chassis melts or which components fail at which temperatures? So I was sort of looking at those weirder, taking-it-to-its-logical-end questions. But I was frustrated because I didn’t have a really practical answer. 

The rice probably doesn’t help most of the time. There are a few people online who do phone repairs who swear that they have the method that is the only good one. But I couldn’t get an authoritative answer, and that itself is kind of a rabbit hole. So I had to pick one rabbit hole to go down. Because I knew that people reading it, a lot of them are going to drop their phones in the water, and I don’t want to give them bad information. I don’t want to be the only source on this. Whereas a problem like how to keep your phone charged, I give lots of impractical advice. But the more practical advice, you can find all over the place. You can find people talking about which power packs you should carry around, tips on reducing battery drain, etc. That seems like it’s a very easy problem to research. How to dry out your phone is kind of a hard problem to research. And I didn’t want to give a bad answer to it.

So you open the book with a disclaimer strongly recommending that people don’t do any of this. Were you concerned that people would try some of the more dangerous but plausible solutions?

That’s something that I think about with How To and that I thought about with What If? I try to avoid stuff that’s on the middle ground of “plausible enough that someone could do it, but also really dangerous.” I try to either talk about stuff that’s very plausible, even if it’s a lot of work, but not all that dangerous, like going to an airport and trying to build a hydroelectric dam that hooks up to the water fountain — which you could do. You might get arrested, but you’ve got no one to blame but yourself if you do that. Then there’s the more dangerous stuff, like installing a lava moat around your house or trying to deliver a package from space. And with those, I tried mostly to tell you all the exciting, terrible things that will happen to you to try. 

“I think that, in a lot of cases, the chapters themselves are very good explanations of why they’re not good ideas.”

So I have a chapter on how to ski where it’s about how to produce snow if you run out of snow ahead of you while you’re skiing — how to carry an apparatus with you to make snow. That one, I think, is going to be pretty hard to pull off. And I think I lay out just how hard and impractical it will be. It’s sort of like a chapter just explaining why you wouldn’t want to do that. And then at the end, it shows all the falling down the hill and the fiery explosions. So I think that, in a lot of cases, the chapters themselves are very good explanations of why they’re not good ideas.

Why the shift from internet comics and blog posts to books for these questions?

When I started out doing comics, I said, “Oh, wow, this is an amazing world. I publish stuff online, and anyone can read it. All of the old ways are dead, and this is the best way to do everything.” So I think, at first, I didn’t really think about books. So essentially, it was a huge surprise to me how much writing a book reached people who I wasn’t reaching otherwise. When I wrote What If? and people kept coming up and telling me that they’d gotten it for their kids and their kids loved it, it sort of never occurred to me. I didn’t realize “Oh, yeah. Kids often want to know more, or are more likely, if anything, to want to know, ‘Hey, can you crash the Moon into the Earth or whatever? What would happen?’” 

And through publishing, through books just being in bookstores, there are a lot of people who might not have stumbled upon my website because they weren’t on the right part of the internet at the right time or whatever, but who have found me through books. And that’s been really cool. 

I also like it because kids ask so much better questions than adults.

Do you have a particular favorite question from the book? 

The “How to be on time” chapter was sort of about all the weird ways that we measure time and how hard it is to keep track of. I’ve always been kind of fascinated by the passage of time and time standards — both from an engineering / programming point of view, like, “How do you reconcile time zones?” and the more physical, like, “What is time, anyway?” in an almost philosophical sense. There’s the “How to tell if you’re a ‘90s kid” chapter, which is sort of on the same topic. So I like diving into those just because I never get tired of thinking about time. But then I think the most fun chapter, almost unexpectedly, was the “How to make an emergency landing” chapter when I interviewed Chris Hadfield. That, I think, ended up being my favorite chapter in the book.