I have a few that I've heard and liked.
(1) The Mobius strippers always show their backside.
(2) Apparently, a quote of Paul Erdos, but it's funny nonetheless : Another roof, another proof.
(3) An experimental physicist meets a mathematician in a bar and they start talking. The physicict asks, "What kind of math do you do?" to which the mathematician replies, "Knot theory." The physicist says, "Me neither!"
(4) The primary reason Bourbaki stopped writing books was the realization that Lang was one single person.