This riddle is a classic. I realized that I've never posted it. Here goes:
You come across a dead body--a man lying face-down with a backpack on his back. Immediately you know how he died. How did the man die?
NOTE: The correct answer has been guessed. To read it, see the comments section.
Lateral Thinking Puzzlers by Paul Sloane has lots of classic riddles, clues and answers. Click here to order it.