QED

Two 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 to the end of the canyon. Someone’s bound to hear us by then!”
So he leans over the basket and screams out, “Helllloooooo! Where are we?” (They hear the echo several times).
Fifteen minutes later, they hear this echoing voice: “Helllloooooo! You’re lost!”
The shouter comments, “That must have been a mathematician.”
Puzzled, his friend asks, “Why do you say that?”
“For three reasons. First, he took a long time to answer, second, he was absolutely correct, and, third, his answer was absolutely useless.”