Rattleback

The Rattleback demonstrates Newton's laws with the flick of a finger. Due to its offset centre of gravity, this scientific curiosity has a preferred direction of rotation: It wants to spin counter clockwise. A rattleback is also known as an "anagyre", "celt", "Celtic stone", "rattlerock", "spin bar" or "wobblestone". It is a semi-ellipsoidal top which will spin on its axis in a preferred direction. But, if spun in the opposite direction, it becomes unstable, "rattles", stops and reverses its spin to the preferred direction. This spin-reversal motion seems, at first sight, to violate the angular-momentum conservation law of physics. Moreover, for most rattlebacks, the motion will happen when the rattleback is spun in one direction, but not when spun in the other.