The yo-yo in its simplest form is an object consisting of an axle

Axle

An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle...

 connected to two disks, and a length of twine

Twine

Twine is a light string or strong thread composed of two or more smaller strands or yarns twisted together. More generally, the term can be applied to any thin cord....

 (usually called a string

String

Generally, string is a flexible piece of rope or twine which is used to tie, bind, or hang other objects.Examples of string use include:* For flying kites* string figures, designs formed by weaving string around one's fingers...

) looped around the axle, similar to a slender spool

Spool

Spool can mean one of the following:*Spool, a usually low-flanged or unflanged cylinder on which thread, wire, cable, paper, film, or tape is wound for distribution or use....

. It is played by holding the free end of the string (usually by inserting one finger in a slip knot

Slip knot

A slip knot is one of two different classes of knot. The most common are knots which attach a line to an object and tighten when tension is applied to the free end of the line...

) allowing gravity or the force of a throw to spin the yo-yo and unwind the string, then allowing the yo-yo's spin to wind itself back to one's hand; Often called "yo-yoing".