In 2014, XKCD creator Randall Munroe released What If?, a book based on his long-running…blog? It’s not his usual “blag,” but I think it qualifies. Anyway, in it, he used science to answer “absurd hypothetical questions” submitted by his readers.

Last month, Randall released What If? 2, in which he answered still more absurd questions…

Q. What if the Solar system was filled with soup out to Jupiter?
A. It would immediately collapse into a black hole.

And sometimes took semi-ordinary questions to absurd lengths…

Q. How long would it take to flood the Earth with puppies (under XYZ conditions)?
A. The observable universe would be filled with dogs in 330 years.

Or sometimes even answered normal questions…

Q. What happens to all the rubber rubbed off of car tires.
A. It’s a major source of microplastics.

Long story short, this book is just as funny and thought-provoking as the original, and absolutely befitting Randall’s style. I said when the first one came out that as a science fiction author, I love this stuff. Even when the scenarios involved are impossible and ridiculous, they lead you down some interesting rabbit holes to learn things that are of use in more practical (often fictional) situations. There’s a long tradition in science of using thought experiments of things we can’t do—that maybe no one can do—to learn more about our universe and the way the real world works, and that applies just as much if not more to envisioning fictional worlds.

There’s not a lot to say about this one, but that’s not for lack of quality. By no means! It’s just that this book is Exactly What It Says on the Tin, and it’s perfect that way. If you like absurd scientific thought experiments, or if you liked the first book, you should definitely read What If? 2.

My rating: 5 out of 5.

P.S. And yes, Randall did leave a few unanswered questions for our entertainment—not as many as last time, but still a good number. Stay tuned for my take on those.