Do you know where we are?

Three guys are flying over a desert in a hot air balloon. They see a man below and yell at him, “Do you know where we are?”

The man thinks about it for a long time and when they’re nearly out of earshot, yells after them, “You’re in a balloon.”

“That was a mathematician,” says one of the aviators. “Why?” asks another. „Because it took him a hell of a long time, the answer is perfectly correct and yet its practical value is zero.”