Opens in a new tab
14 min 25 sec
Available in:Englishไทย

What If? 2: Additional Serious Scientific Answers to Absurd Hypothetical Questions

Randall Munroe takes ridiculous reader questions, from a house-sized Jupiter to eating a cloud, and answers them with real physics and math. Along the way he shows how to test assumptions and estimate almost anything.

Table of Content

If science class left you thinking physics is a wall of formulas, Randall Munroe offers a cheerful way back in. In What If? 2, the creator of xkcd and former NASA robot builder answers a reader who asked how many people a Tyrannosaurus rex would have to eat each day to meet its calorie needs. His estimate is about half an adult, or one ten-year-old, or roughly 80 Big Macs if a McDonald’s is nearby.

Since 2012, Munroe has invited readers to send him the most ridiculous hypotheticals they can dream up, and he treats every one as a serious problem. His argument is that a silly question, worked through honestly, builds the habits serious thinking needs: checking hidden assumptions, noticing what breaks when scale changes, and estimating with numbers you already trust. Six of his answers, from a house-sized Jupiter to an edible cloud, show those habits in action, and each one gives you a reasoning move for the next confident claim you hear at work.

A house-sized Jupiter would weigh only about 2,500 tons, not enough to form a black hole and disturb the neighborhood’s gravity. The trouble lies somewhere far less obvious, and it shows up again in any system you try to resize.

Everyone uses the word snowballing to mean growth that keeps speeding up. Munroe rolled an imaginary snowball down the world’s highest mountain and found the metaphor points the wrong way.

Extend every country’s borders straight up into space and one nation ends up controlling more of the Milky Way than any other. The winner is not the biggest country, and the reason says a lot about outcomes in general.

The old saying insists Rome was not built in a day. With a construction formula, a guess at property values and Michelangelo’s painting speed, Munroe shows how long it would really take, and how to estimate anything similar.

A cloud is only air and water, so eating one sounds possible, if pointless. Munroe found a surprising reason it fails, and it is the kind of flaw that hides inside many sensible-looking plans.

Standing in an unbreakable glass tube at the deepest point of the ocean sounds like a rare view. Munroe explains why it would be cold, dark and very hard to leave, and how to escape alive.

Munroe’s case is that silliness and rigor work well together. Every question looked like a joke, and every answer turned on a real idea: gravity holding a planet’s heat in place, surface area slowing growth, the tilt of the Earth, a burp that rebuilds the cloud you just ate. The fun gets you in the door, and the reasoning is what you keep. Munroe ends on exactly that note, that chasing answers to absurd questions exercises the analytical mind while staying enjoyable.

The habits travel easily into ordinary weeks. When a plan or forecast lands on your desk, look for what holds the current situation together and whether it survives a change in size. When someone says a number will keep snowballing, ask what has to keep growing for that to stay true. When a question seems impossible, break it into two or three figures you can guess and offer a range instead of one confident number. And before committing to anything deep, sketch the way back up.

The T. rex question has an answer, about 80 Big Macs a day. The lasting gift is the confidence that almost any question has one, if you are willing to work it out.

About this book

What is this book about?

Plenty of people left school believing physics and math were dull and out of reach. Randall Munroe, the physicist behind the webcomic xkcd and a former NASA robot builder, has spent years arguing the opposite. Starting in 2012, he invited readers to mail in the weirdest what-if scenarios they could imagine, and he treats every one with the care of a genuine research problem. What If? 2 collects more of those answers. Could you eat a cloud? What would happen if Jupiter shrank to the size of a house and landed on your street? How many people would it take to build Rome in a day? Each answer is funny, and each turns on a genuine scientific idea: gravity holding back heat, surface area slowing growth, the tilt of the Earth, the hidden side effect that ruins a plan. Readers get the answers and also a way of thinking. Munroe shows how to break an impossible question into a few numbers you can guess, check them against a known benchmark, and spot the assumption that makes the obvious first answer wrong.

Book Information

Rating:

Genre:

Science

Topics:

Critical Thinking, Physics

Publisher:

Penguin Random House

Language:

English, ไทย

Narrator:

Meredith

First published:

September 13, 2022

Pages:

368

Length:

14 min 25 sec

Get the book:

Amazon

Who it's for

  • Bored by school science

    Curious readers who left school thinking physics and math were dull and want a playful way back in.

  • Needing quick estimates

    Analysts, managers and planners who need to estimate quickly and spot weak assumptions.

  • Wanting more reasoned nonsense

    Fans of xkcd and the first What If? looking for more seriously reasoned nonsense.

Listen to Preview

About the Author

Randall Munroe

Randall Munroe was born in Easton, Pennsylvania, and grew up near Richmond, Virginia. He studied physics at Christopher Newport University and then built robots at NASA Langley Research Center. In 2006 he left NASA to draw comics full time and has produced xkcd ever since. His work has earned Hugo Award nominations, and the asteroid 4942 Munroe carries his name. He also wrote What If?, Thing Explainer and How To, and What If? and Thing Explainer were New York Times bestsellers.

Ratings & Reviews

Ratings at a glance

4.1

Overall score based on 213 ratings.

What people think

Listeners find this book to be a delightful and captivating experience, featuring incredibly witty content that fuses comedy with scientific insight. It offers unexpectedly thorough research into ridiculous inquiries, quenching curiosity with numerous trivia points and serving as a perfect present for enthusiasts or younger audiences. Listeners value its accessible and stimulating quality, with one listener mentioning that it is simple to grasp.

Top reviews

Dome

Picked this up after the first one blew my mind, and Randall Munroe is still the king of making absolute chaos sound perfectly logical. I spent an entire hour laughing about the logistics of filling the solar system with tomato soup and how it would inevitably turn into a massive black hole. As a physics nerd, I appreciate that he doesn’t skip the math, yet he manages to keep it accessible for people who haven't touched a calculator since high school. The stick figures are as charming as ever, providing the perfect visual punchline to scenarios that would otherwise be terrifying, like the Earth spinning so fast it shreds itself. It’s the ultimate coffee table book for curious minds. Truth is, you don’t even need to read the first one to dive into these chapters. It’s just pure, unadulterated nerd joy that explains the 'why' behind the 'what' in the most hilarious way possible.

Show more
Lincoln

Wil Wheaton’s narration on the audiobook version is an absolute masterclass in geeky delivery. He clearly loves the material, and hearing him chuckle during the more ridiculous segments about pigeons lifting skyscrapers made the whole experience feel like a conversation with a funny friend. I did have to grab the ebook as well just to see the comics, because you really can’t get the full Randall Munroe experience without those iconic stick figures. The way he breaks down complex planetary motion or gravitational pull into bite-sized, hilarious scenarios is brilliant. It makes science feel approachable and, more importantly, fun. Whether you're a hardcore science buff or just someone who likes to imagine what would happen if the moon were made of cheese, this book delivers. It's rare to find something that is simultaneously this educational and this entertaining.

Show more
Somrutai

Finally got around to reading this and I can confirm that Randall Munroe is a national treasure. The sheer amount of effort that goes into answering questions like "What if Earth was a massive eyeball?" is staggering. Most people would just give a joke answer, but he actually sits down and calculates the biological and gravitational implications. Frankly, it’s the most fun I’ve had with a non-fiction book in years. The prose is snappy, the jokes in the footnotes are top-tier, and the drawings add that perfect xkcd flair. It’s one of those rare books that you can open to any random page and immediately be sucked into a fascinating rabbit hole of science. It’s incredibly readable for such a data-heavy book, which is a testament to Munroe’s skill as a writer. If you have even a passing interest in how the universe works, buy this immediately.

Show more
Bunyarit

Not what I expected for a 'science' book, but I am absolutely obsessed with the tone of this. It’s basically what happens when you give a brilliant physicist a bunch of crayons and some very weird emails from the internet. The scenarios are so absurd—like the soup-filled solar system—that you can't help but laugh out loud. I started reading a few chapters to my nephew, and now he’s obsessed with the idea of a T-Rex roaming New York. It’s a brilliant way to get kids interested in physics without them even realizing they’re learning. Personally, I found the footnotes to be the best part; they’re packed with extra jokes and snarky comments that show Munroe’s personality. It’s lighthearted, deeply researched, and just plain fun. If you enjoyed the first one, there’s no reason to hesitate on this. It’s a five-star follow-up.

Show more
Sam

Randall Munroe has this incredible knack for taking the most ridiculous premises and treating them with absolute scientific reverence. Whether he’s discussing the catastrophic effects of speeding up Earth's rotation or the caloric needs of an urban dinosaur, he never talks down to the reader. The truth is, I learned more about centrifugal force and black holes in two chapters of this than I did in a semester of college physics. The humor is dry and perfectly matched with the minimalist stick-figure art style we all know from xkcd. It’s such a satisfying read because it rewards curiosity with actual data, even if the data is about how tomato soup would crush the universe. It’s an amazing gift for any nerd, but it’s also just a great book for anyone who wants to see the world from a slightly skewed, much more interesting perspective.

Show more
Sangduan

The chapter on T-Rex calorie intake was exactly the kind of weirdly specific content I was looking for. Randall has this unique ability to take a question that sounds like it was asked by a toddler on a sugar high and treat it with the rigorous scrutiny of a NASA scientist. While I loved the humor and the xkcd-style illustrations, there were a few moments where the calculations went a bit over my head. To be fair, I’m not a bioengineer, so I found myself skimming the heavier physics blocks to get back to the funny footnotes. Even with the occasionally dense jargon, the book is incredibly thought-provoking and stays grounded in real-world logic. It’s a great gift for anyone who likes 'useless' facts that are actually deeply fascinating. I do wish some of the shorter answers had a bit more meat on their bones, but it's still a solid follow-up to the original.

Show more
Pear

Ever wonder what happens when you use an arc welder as a defibrillator? According to this book, you probably shouldn't even be allowed to own the welder in the first place! That kind of dry, witty humor is what makes "What If? 2" such a joy to read. It’s effectively a manual for things you should never do, explained through the lens of a brilliant former NASA roboticist. The research is surprisingly deep, covering everything from relativistic speeds to how many bones you can lose and still survive. It’s a fantastic gift for kids who are constantly asking "why?" or for the adult nerd in your life who has everything. My only tiny gripe is that a few of the questions felt like retreads of themes from his blog, but the new illustrations make it worth the price. It’s light, engaging, and will definitely make you the most interesting person at your next dinner party.

Show more
Prapaiwan

This book made me feel incredibly smart and incredibly dumb at the same time, which is quite an achievement. I love the wacky, science-based approach to things like Japan leaving the planet or pigeons causing a gravitational shift. Randall’s writing style is so engaging that I found myself nodding along to math I barely understood. To be fair, some of the sections on relativistic collisions are a bit dense, but the payoff is usually a hilarious stick-figure drawing of a catastrophic explosion. It’s a wonderful example of how to make STEM education actually exciting for the general public. While it’s essentially more of the same from the first book, I don't see that as a bad thing at all. It’s a money machine that works because the content is genuinely high-quality. Great for a quick laugh or a long, deep dive into the 'what if' scenarios that keep us up at night.

Show more
Sayan

After hearing so many people rave about Munroe's blog, I decided to jump straight into this sequel. The good news is that you don't need to have read the first "What If?" to enjoy this one. Each chapter is a standalone piece of scientific madness. I was particularly fascinated by the section on how many pigeons it would take to lift someone, though the math did get a bit dizzying toward the end. Not gonna lie, I was a little bummed by a few of the 'short answers' which felt like they ended right as the science was getting juicy. However, the overall package is so entertaining that it’s hard to complain too much. The book is physically beautiful and the illustrations are iconic for a reason. It’s a solid 4-star read that will definitely satisfy your curiosity about the weirdest corners of reality.

Show more
Ning

As someone who loved the first book, I found this sequel to be a bit of a mixed bag. The humor is definitely still there, and Munroe’s imagination is clearly boundless, but some of the explanations felt much more technical this time around. Look, I’m a nurse, and I’m used to explaining complex medical concepts in simple terms, but I found myself getting lost in the weeds of the physics equations here. I really wanted to understand why the solar system soup scenario ends time, but the breakdown was a little too jargon-heavy for a casual reader like me. The short answers were also a bit disappointing because they felt like they ended right when they were getting interesting. It’s still a fun read for the cartoons and the general vibe, but it lacked the perfect balance that made the first one so accessible.

Show more
Show all reviews

Questions about this book

It collects Munroe’s answers to absurd hypothetical questions sent in by readers, such as eating a cloud, building Rome in a day or shrinking Jupiter to the size of a house. Each answer uses real physics and math. Underneath the humor, the book shows how to reason carefully: question the premise, notice what changes with scale and estimate with rough but sensible numbers.

Munroe first notes that adding workers creates problems of training, crowding and supply. Playing along, he estimates about 2 billion hours of labor, so eight billion people could finish in little more than 15 minutes. If every square meter were done at Michelangelo’s Sistine Chapel pace, it would take about two and a half hours.

Not directly, Munroe explains. You can swallow a cloud’s water, but you also take in air, which you burp back out warm and moist. That breath condenses into new cloud, so you make cloud faster than you eat it. If you extract the water first, a house-sized cloud holds 2 to 3 liters, about what a stomach can comfortably hold.

The answers model useful thinking habits. A house-sized Jupiter shows that a stable system may depend on a force that disappears when you change its scale. The Everest snowball shows that growth often slows rather than speeds up. The Rome estimate shows how to break a hard question into a few guessable numbers and give a range.

It suits curious readers who want science explained through humor, people who felt shut out by physics and math at school, and professionals who estimate projects or test plans for weak spots. Fans of the xkcd webcomic and of Munroe’s first What If? will recognize the same mix of jokes and careful calculation.

AUDIO SUMMARY AVAILABLE

Listen to What If? 2 in 15 minutes

Get the key ideas from What If? 2 by Randall Munroe, plus 5,000+ more nonfiction titles.

  • 5,000+ titles
  • Listen as much as you want
  • 7-day free trial
  • Cancel anytime

What If? 2

Preview · Full summary 14 min 25 sec
Home

Discover

Open app

Log in