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:''For the fictional animal, see [[Rattleback (rodent)]]''.
[[Image:Celt_with_gemstone_turtles-01.jpg|thumb|
A '''rattleback''', also known as a "celt," "Celtic stone," "rattlerocks," "spin bar," "Tate's," "wobble stone" or "wobblestone," and by the product names "ARK," "RATTLEBACKS" and "Space Pets," 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.
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The spin-reversal motion follows from the growth of [[flight dynamics|instabilities]] on the other rotation axes, that are rolling (on the main axis) and pitching (on the crosswise axis).
[[Image:Rolling-pitching.png|thumb|200px|Rolling and pitching motions]]
When there is an asymmetry in the mass distribution with respect to the plane formed by the pitching and the vertical axes, a coupling of these two instabilities arises; one can imagine how the asymmetry in mass will deviate the rattleback when pitching, which will create some rolling.
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