There's always the Riddle of the Sphinx:
What walks on four legs in the morning,
Two legs at Noon,
And three legs in the Evening?
-OR-
How can bullets bounce off Superman, but yet he ducks when you throw a gun at him?
How many years was the 100 Years' War?
How many software engineers does it take to change a light bulb?
(None, it's a hardware problem... )
What exactly is a jumbo shrimp?
How did the chickpea get it's name? It's neither a chick nor a pea.
Oh, and here's one that seems a little backwards: Why do you "take a dump?" Wouldn't you rather get rid of it?
Duane <-tongue in cheek