Astrojax

Astrojax, invented in 1986 by Larry Shaw, is a toy consisting of three balls on a string. In the original version of the toy, one ball is fixed at each end of the string, and the center ball is free to slide along the string between the two end balls. Inside each ball is a metal weight. The metal weight lowers the moment of inertia of the center ball so it can rotate rapidly in response to torques applied by the string. This prevents the string from snagging or tangling around the center ball.

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