A yo-yo (also spelled yoyo) is a toy consisting of an axle connected to two disks, and a string looped around the axle, similar to a spool. It is played by holding the free end of the string known as the handle (by inserting one finger—usually the middle or index finger—into a slip knot), allowing gravity (or the force of a throw and gravity) to spin the yo-yo and unwind the string (similar to how a pullstring works). The player then allows the yo-yo to wind itself back to the player's hand, exploiting its spin (and the associated rotational energy). This is often called "yo-yoing".