And one more...
A mathematician and an engineer are called in for a psychological experiment. They're sat down in two chairs at the end of this incredibly long room. At the end, a door opens and a naked woman steps out. The mathematician and engineer squint, and even then can just barely see her.
An intercom on the wall announces, "at the sound of the bell, you may move your chair half the distance between you and the naked lady."
So the bell rings, and the engineer scootches halfway down the entire hallway (mime out scootching here for added laughs when telling the joke). However, the mathematician doesn't move.
Again, the bell rings and the engineer scootches halfway between him and the woman, but the mathematician doesn't move.
The experiment is stopped and one of the scientists behind the experiment says to the mathematician, "the instructions were clear! Why didn't you move?" The mathematician replies, "I'm not an idiot. Obviously if we only move half the distance, we'll never get to the naked woman."
So the scientist says to the Engineer, "Well if that's true, why did you move?" The engineer replies, "Well, I knew that, but I figure I'll get close enough for practical purposes."