On his "bountiful" web site, Erich Friedman, math professor at Stetson University (Deland, FL), includes a wide collection of "Top Ten" lists. One of those is his "Top Ten Favorite Math Riddles." You have been 4-warned...
Question: Why did the mathematician name his dog "Cauchy"?
Answer: Because he left a residue at every pole.
Question: What's sado-masochism?
Answer: The standard deviation of the mean.
Question: What do you get when you cross an elephant and a grape?
Answer: I dunno, but its magnitude is Elephant, grape, sin theta.
Question: What do you get when you cross a mountain climber with a grape?
Answer: You can't. A mountain climber is a scaler.
Question: What do farmers study in trigonometry?
Answer: Swine and cow-swine.
Question: What's the contour integral around Western Europe?
Answer: Zero, because all the Poles are in Eastern Europe.
Question: How many numerical analysts does it take to screw in a light bulb?
Answer: 0.9973 after the first three iterations.
Question: How many statisticians does it take to change a lightbulb?
Answer: Two plus or minus three.
Question: How many applied mathematicians does it take to screw in a lightbulb?
Answer: One, who gives it to two statisticians, thereby reducing it to an earlier riddle.
Question: How many topologists does it take to change a light bulb?
Answer: It really doesn't matter, since they'd rather knot.