Rattleback (Celtic Stone).

The Rattleback is a long and thin half elliptical shape formed in plastic so that it has a slightly offset center of gravity. Also known as the Celtic stone, or wobble stone it is a fascinating object. You spin it in one direction, it slows down, wobbles, and then starts spinning in the opposite 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. Some exceptional rattlebacks will reverse when spun in either direction. This makes the rattleback a physical curiosity that has excited human imagination since prehistorical times.