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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" to a stop and reverses its spin to the preferred direction.
This spin-reversal appears to violate the law of the [[angular momentum#Conservation of angular momentum|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.<ref name="motivate">{{cite web|title=Introduction to Hugh's Talk |work=Millennium Mathematics Project |url=http://motivate.maths.org/conferences/conf14/c14_talk1.shtml |archive-url=https://web.archive.org/web/20040306062339/http://www.motivate.maths.org/conferences/conf14/c14_talk1.shtml |
This makes the rattleback a physical curiosity that has excited human imagination since prehistorical times.<ref>"celt, n.2". OED Online. September 2012. Oxford University Press. 1 October 2012 <http://www.oed.com/view/Entry/29533?isAdvanced=false&result=2&rskey=EPfrjA&></ref>
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