The esteemed Notices of the AMS (January 2005) included an article by Paul Renteln and Alan Dundes, entitled "Foolproof: A Sampling of Mathematical Folk Humor." Below is an offering that ranges from analysis (complex/real) to group theory to linear algebra to number theory to ....Again, Enjoy or Boo & Hiss or even Scratch Your Head in Puzzlement!
Q: What do you call a young eigensheep?
A: A lamb, duh!.
Q: What's the value of a contour integral around Western Europe?
A: Zero, because all the Poles are in Easter Europe.
Addendum: Actually, there ARE some Poles in western Europe, but they are removable!
Q: Why did the mathematician name his dog "Cauchy"?
A: Because he left a residue at every Pole.
Q: What's a topologist?
A: Someone who cannot distinguish between a doughnut and a coffee cup.
Q: Why didn't Newton discover group theory?
A: Because he wasn't Abel.
Q: What do you get if you cross an elephant and a banana?
Q: What do you get if you cross a mosquito with a mountain climber?
A: You can't cross a vector with a scaler.
Q: What do you get when you cross a mountain goat with a mountain climber?
A: Nothing--you can't cross two scalers.
Q: What's a polar bear?
A: A rectangular bear after a coordinate transform.
Q: What's can't you grow wheat in Z/Z6?
A: It's not a field.
Q: What's big, grey, and proves the uncountability of the reals?
A: Cantor's diagonal elephant.
Q: What's gray and huge and has integer coefficients?
A: An elephantine equation.