Bad Math Joke

We're currently having a bad math joke thread (with pretty frequent e-mails) on the Project NExT list. My contribution was basically to provide a link to an article that appeared a few years ago in Notices of the American Mathematical Society that collected a bunch of them.

Most of the jokes in the current thread have been bad and a few of them have been so horrible that they've made me groan, but here is a slightly reworded one that I actually really like:

Question: How can you tell that a guy is a topologist?

Answer: He can't tell his ass from a hole in the ground, but he can tell his ass from two holes in the ground.