Ever heard of downloading eps off the net? Seen this one ages ago. Will not spoil it.
The best one is a few more weeks "the airplane on a conveyor belt":
"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?"
So if the belt is matching the take off speed (in the opposite direction to travel) of a lightie, will it take off?