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 far." So he leans over the basket and yells out, "Hellllooooo! Where are we?" (They hear the echo several times). Two minutes later they hear this echoing voice: "Hellllooooo! You're lost!!" One of the men says, "That must be a mathematician." Puzzled, one of the other men asks, "Why do you say that?" He replies: "For three reasons. (1) he took a long time to answer, (2) he was absolutely correct, and (3) his answer was absolutely useless."