Mathematikerwitze und Physikerwitze - Witz Nr. 125
What's purple and commutes? An abelian grape.
What's yellow and equivalent to the Axiom of Choice? Zorn's Lemon.
What does the little mermaid wear? An algae-bra.
How does a mathematician call his dog? Cauchy - because it leaves a residue at every pole...
What's the contour integral around Western Europe? Zero, because all the Poles are in Eastern Europe!
Addendum: Actually, there are some Poles in Western Europe, but they are removable!