For over two years now, Randall Munroe has, in his own words, been "Dear Abby for mad scientists", answering questions like 'what happens when you hit a baseball travelling at 90% of the speed of light?'
Munroe is both an ex-NASA physicist and the creator of the wildly-popular webcomic XKCD (http://www.xkcd.com), and both his scientific chops and trademark humour are on full display as he contemplates putting submarines into Earth-orbit, draining Earth's oceans and transporting them to Mars, and using machineguns as a source of thrust.
Few of these end well for the experimenters
For my money, some of the most hilarious entries are the questions that he refuses to answer - such as how hot a chainsaw would need to be in order to cauterise human flesh as it passed through it ("...I need to know by Friday."), or whether humans currently have the technology to turn the sun off and on ("...I need to know by Friday.").
Whew, that was close!
The UK edition, which I read, includes a special foreword where Munroe compares the clarity of the metric system to the strangeness of the imperial systems used by the US and UK in daily life. He then goes on to suggest improving the imperial system... by making it vastly more complicated.
What If? comes highly recommended for anyone with a love of science and a sense of fun.