Because I'm bored, here's a brain teaser for your 4th of July weekend:
A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same, but in the opposite direction. Can the plane take off?
Update: Answer on page 2 (and also below).