xkcd's a webcomic created by an ex NASA scientist.
Hilarious, sometimes seriously perverted by a mind trick, it's always a pleasure to read its strips.
For those who don't get it all, there's a wiki called explainxkcd.com.
Then, a new section was created : what if...
Purpose was to seriously answer to dumb questions.
Like : What would happen if the moon were replaced by a black hole ?
And what must happen happened : a wild book appears.
it contains 51% unpublished topics, and is a must have for any xkcd lover.