A rattleback, also known as a wobblestone or a celt, is a type of top that has a preferred direction to spin in. That direction varies based on the particular rattleback and can either be clockwise or counterclockwise. If spun in the unpreferred direction, the top will stop and begin to spin in the preferred direction. Rattlebacks are an ancient toys that have fascinated people for thousands of years primarily because of its apparent defiance of the laws of physics.
A rattleback is semi-ellipsoidal in shape — flat on top and curved on the bottom, and significantly longer than it is wide. Ancient rattlebacks were typically made of stone, but today they may be made of nearly any hard material, including plastic, wood, and glass. One can even be made out of a wrapped stick of chewing gum. 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.
No matter the direction a rattleback is spun — whether it's the "right" direction or "wrong" one — it'll spin in a bit of an unstable manner. When spun in the unpreferred direction, it'll wobble slow down and start spinning in the preferred direction; but even when it spins in the preferred direction, it'll spin well for a while, wobble and stop spinning. A wooden or stone rattleback will 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 spinning in one direction to continue spinning in the same direction until some force intervenes to stop it. 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 many with their surprising mid-spin reversal of motion.