Yo-yo

A Yo-yo in its' simplest form is an object consisting of an axle connected to two disks, and a length of string looped around the axle, similar to a slender spool. It is played by holding the free end of the string (usually by inserting one finger in a slip knot) allowing gravity or the force of a throw to spin the yo-yo and unwind the string (similar to how a pullstring works), then allowing the yo-yo to wind itself back to one's hand, exploiting its spin (and the associated rotational energy). Colonel Jack O'Neill has been known to play with a yo-yo.