A rattleback, also known as a "celt," "Celtic stone," "rattlerock," "spin bar," "anagyre," "Tate's," "wobble stone" or "wobblestone," and 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. Behold the mysterious celt,with a property that amuses. One way it will spin, the other way it refuses. This spin-reversal motion seems, at first sight, to violate the angular-momentum conservation law of physics. Moreover, for most rattlebacks, the motion will happen when the rattleback is spun in one direction, but not when spun in the other. These two peculiarities make the rattleback a physical curiosity that has excited human imagination since prehistorical times.