Here's a classic, that some of you may have already heard:
A man lives on the 20th floor of a building. Every morning he gets up and takes the elevator down to the first floor and goes to work. When he returns home from work, he takes the elevator only to the 10th floor and then walks the stairs the rest of the way. Except when it's raining. Then he takes the elevator all the way up to the 20th floor. Why?
NOTE: The correct answer to the riddle has been guessed. Please see the comments section for the answer.
Lateral Thinking Puzzlers by Paul Sloane has lots of classic riddles, clues and answers. Click here to order it.