A rattleback, also known as a wobblestone or a celt, is a type of top that will only spin in one direction. If spun in the other direction, the top will stop and begin to spin in the preferred direction. The rattleback is an ancient toy that has fascinated humans for thousands of years for its apparent defiance of the laws of physics.

A rattleback is semi-ellipsoidal in shape, flat on top and round on the bottom, and significantly longer than its width. Ancient rattlebacks were typically made of stone, but today they may be made of nearly any hard material, including plastic, wood, and glass. The rattleback works because of some sort of asymmetry, either in the base itself or in the form of asymmetrical weights attached to the top of the base. This asymmetry causes the rattleback to become unstable when spun in one direction.

While a rattleback will quickly become unstable, wobble, and reverse its spin direction when spun in the direction which is not preferred, both directions of spin are unstable. However, the top can spin in the preferred direction for longer without starting to wobble and reversing motion. A wooden or stone rattleback with typically slow down due to friction before becoming unstable in the preferred spin direction. A glass rattleback, however, may switch spin directions a few times before slowing down.

Rattlebacks seem mysterious because people expect something moving in one direction to continue moving in the same direction until some force intervenes to stop it — a law which physics terms angular-momentum conservation. For this reason, rattlebacks have sometimes been considered supernatural and used as a means of divination. Many physics papers have been published on the subject of the rattleback, and most people today recognize that it is simply a toy and that its apparently anomalous movement has a rational explanation. Nevertheless, rattlebacks continue to amuse with their surprising mid-spin reversal of motion.