A mathematician and an engineer are sitting next to each other on a long flight. The mathematician leans over to the engineer and asks if he would like to play a fun game. The engineer just wants to take a nap, so he politely declines and rolls over to the window to catch a few winks.
The mathematician persists and explains that the game is real easy and lots of fun. He explains, "I ask you a question, and if you don't know the answer, you pay me £5. Then you ask me a question, and if I don't know the answer, I'll pay you £5."
Again, the engineer politely declines and tries to get to sleep. The mathematician, now somewhat agitated, says, "Okay, if you don't know the answer, you pay me £5, and if I don't know the answer, I'll pay you £50!"
This catches the engineer's attention, and he sees no end to this torment unless he plays, so he agrees to the game. The mathematician asks the first question. "What's the distance from the earth to the moon?"
The engineer doesn't say a word, but reaches into his wallet, pulls out five pounds and hands it to the mathematician Now, it's the engineer's turn. He asks the mathematician "What goes up a hill with three legs and comes down on four?"
The mathematician looks up at him with a puzzled look. He takes out his laptop and searches all of his references. Frustrated, he sends e-mail to his co-workers but to no avail.
After about an hour, he wakes the engineer and hands him £50. The engineer politely takes the £50 and turns away to try to get back to sleep.
The mathematician then hits the engineer, saying, "What goes up a hill with three legs, and comes down on four?" The engineer calmly pulls out his wallet, hands the mathematician five pounds, and goes back to sleep.