A rattleback, also known as an "anagyre", "celt", "Celtic stone", "rebellious celt", "rattlerock", "spin bar", "wobble stone" or "wobblestone" an by the product names "ARK," "Bizzaro Swirls," "RATTLEBACKS," "Space Pet" and "Space Toy," 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.