A rattleback or wobblestone is an object with a smooth bottom that can be spun like a top. Unlike conventional tops, however, the rattleback has an asymmetry which gives it a preferred direction of spin. If spun in the opposite direction, it will begin to wobble, and then change the direction of its spin.
It would seem that if a spinning object changes the direction of its spin, it is defying the law of conservation of angular momentum, but apparently it is the rocking/wobbling motion that is converted into counter-rotational motion, not the initial rotation.
From the Upcoming ueue, submitted by Minnesotastan.