please tell me the most obscure joke you know

(feel free to explain or not explain it, depending on what you find more amusing to think of me reading it)

@whitequark Q: What's an anagram of Banach-Tarsky?

A: Banach-Tarsky Banach-Tarsky.

@whitequark Oh and of course:

Q: What's the shortest math joke?
A: Let ε < 0.

Being that ε, by convention, is always a very small number greater 0, and it being smaller (or equal, in some variations) is just a startling effect.