why did the chicken...
Q: Why did the chicken cross the Möbius strip?
A: To get to the other... uh...
(taken from here)
Bonus (even worse):
ex and k are walking down a city street.
"Oh no," says k, "there's a differential operator coming. I'm a constant: if he differentiates me, there'll be nothing left!"
"Ah," says ex, "he won't bother me. Watch this!"
ex comes up to the differential operator.
"Hi, I'm ex."
"Hi! I'm d/dy."
("Ha! Only one person in a million would find that funny.")