Two men are in a hot air balloon floating over the countryside.
They are lost.  They see a man far below them and scream down to
him "where are we?"  After about a minute and a half, the man
shouts back "You are in a hot air balloon!"

One of the men in the balloon tells his friend "that must be a
mathematician."

"How do you know?" , the other asks.

"Three reasons:
          1) He took a while to answer.
          2) He was absolutely correct
          3) His answer didn't help me in the least!"