A yo-yo (also spelled as yoyo) is a toy consisting of an axle connected to two disks, and a string looped around the axle. It has some similarities to a slender spool. A yo-yo 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". In the simplest play, the string is intended to be wound on the spool by hand; The yo-yo is thrown downwards, hits the end of the string, then winds up the string toward the hand, and finally the yo-yo is grabbed, ready to be thrown again. One of the most basic tricks is called the sleeper, where the yo-yo spins at the end of the string for a noticeable amount of time before returning to the hand.