What If? Serious Scientific Answers to Absurd Hypothetical Questions, by Randall Munroe
Lots of people read Munroe's webcomic xkcd--I do--and if you don't, you should probably take a peek at it to see if you'd like it. It's full of geeky fun. Munroe also writes a column called What If? in which you can send in a ridiculous question and he'll answer it in great detail. For example:
What if I collected a mole of moles?
What if a baseball was thrown at 90% of the speed of light?
What if I collected all the elements and arranged them according to the periodic table?
What if I dove into a spent nuclear fuel pool?
How long could a nuclear submarine last in orbit?
What if we all shone laser pointers at the Moon?
What if we built a bridge out of Legos?
Many of the answers to these questions are some variation on "we would all die horribly," so the fun is in the details. (You would not die if you dove into a spent nuclear fuel pool; technicians do that for maintenance. Just don't get too close to the actual material.)
So this book is a collection of many of those What If? columns. They are very fun to read, and my 12yo daughter zipped through it in about a day, giggling all the while. I found that I got tired towards the end of the book; this is more fun to dip into than it is to read all at once. And, sadly, the cover illustration is entirely hypothetical; no Tyrannosaurus Rex vs. Sarlacc scenario is included. But it's a fun read, and I know I'll be lending it out to friends.