Threesome
Permanent link to this comic: http://xkcd.com/613/
Image URL (for hotlinking/embedding): http://imgs.xkcd.com/comics/threesome.png
Woman: We had a threesome last night.
Man: How was it?
Woman: Awkward -- it was with a physicist.
Man: Why's that awkward?
Woman: They can't solve the three-body problem.
Man: Ah, yes.
{{Title text: I wanted us to try finding an approximate numeric solution, but noooo.}}