This wooden box has a switch on it. When the switch is flipped on, the box flips it off again. On, off. On, off. On….off. And then the box gets more creative in making sure the switch remains off.

(via @davidfg)

Update: Claude Shannon had such a box on his desk, although probably not so aggro as the one above. It was called the Ultimate Machine. (thx, @joshu)