What is "rattleback"

n. A form of top (toy) that will spin preferentially in one direction (rattling, then reversing, if spun in the other direction)

A rattleback is a semi-ellipsoidal top which will rotate on its axis in a preferred direction. If spun in the opposite direction, it becomes unstable, "rattles", stops and reverses its spin to the preferred direction.

This spin-reversal appears to violate the law of the conservation of angular momentum. 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.

A rattleback may also be known as a "anagyre", " celt", "Celtic stone", "rebellious celt", "rattlerock", "spin bar", "Robinson Reverser", "wobble stone" (or "wobblestone") and by the product names "ARK", "Bizzaro Swirl", "RATTLEBACK", " Space Pet" and "Space Toy".