Three men are in a hot-air balloon. Soon, they find themselves lost
in a canyon somewhere. One of the three men says, "I've got an idea.
We can call for help in this canyon and the echo will carry our voices
So he leans over the basket and yells out, "Helllloooooo! Where are
we?" (They hear the echo several times.)
15 minutes later, they hear this echoing voice: "Helllloooooo! You're
One of the men says, "That must have been a mathematician."
Puzzled, one of the other men asks, "Why do you say that?"
The reply: "For three reasons. (1) he took a long time to answer, (2)
he was absolutely correct, and (3) his answer was absolutely useless."