A yo-yo (also spelled 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.
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.
A Greek vase painting from 440 BC shows a boy playing with a yo-yo (see right). Greek records from the period describe toys made out of wood, metal, or painted terra cotta (fired clay). The terra cotta disks were used to ceremonially offer the toys of youth to certain gods when a child came of ageŽdiscs of other materials were used for actual play.
The principal distinction between the Filipino design popularized by Flores and more primitive yo-yos is in the way the yo-yo is strung. In older (and some remaining inexpensive) yo-yo designs, the string is tied to the axle using a knot. With this technique, the yo-yo just goes back-and-forth; it returns easily, but it is impossible to make it sleep. In Flores's design, one continuous piece of string, double the desired length, is twisted around something to produce a loop at one end which is fitted around the axle. Also termed a looped slip-string, this seemingly minor modification allows for a far greater variety and sophistication of motion, thanks to increased stability and suspension of movement during free spin.
In a trademark case in 1965, a federal court's appeals ruled in favor of the Royal Tops Company, determining that yo-yo had become a part of common speech and that Duncan no longer had exclusive rights to the term. As a result of the expenses incurred by this legal battle as well as other financial pressures, the Duncan family sold the company name and associated trademarks in 1968 to Flambeau, Inc, which had manufactured Duncan's plastic models since 1955. As of 2014, Flambeau Plastics continued to run the company.
Swedish bearing company SKF briefly manufactured novelty yo-yos with ball bearings in 1984. In 1990, Kuhn introduced the SB-2 yo-yo that had an aluminum transaxle, making it the first successful ball-bearing yo-yo.
There are many new types of ball bearings in the market which deviate from the original design and/or material of the standard stainless steel ball bearing. For example, a certain type of bearing has an inward facing curved surface, to prevent the string from rubbing on the sides of the yo-yo, which would cause unwanted friction when performing intricate string tricks. Other manufacturers replicate this with a similar inwardly curved surface, but use minor modifications. Some high-end bearings use ceramic composites in the balls of the bearing, to reduce internal friction, again making for a smoother spinning yo-yo.
The essence of the throw is that one throws the yo-yo with a very pronounced wrist action so that when the yo-yo reaches the end of the string it spins in place rather than rolling back up the string to the thrower's hand. Most modern yo-yos have a transaxle or ball bearing to assist this, but if it is a fixed axle yo-yo, the tension must be loose enough to allow this. The two main ways to do this are (1), allow the yo-yo to sit at the bottom of the string to unwind, or (2) perform lariat or UFO to loosen the tension. When one decides to end the "sleeping" state, one merely jerks the wrist and the yo-yo "catches" the string and rolls back up to the hand. Ball-bearing yo-yos with a "butterfly" shape, primarily used for string tricks, frequently (but not always) have the low response (or are, in fact, completely unresponsive), requiring a "bind" for the yo-yo to return.
Yo-yos optimized for looping have weight concentrated in their centers so they may easily rotate about the string's axis without their mass contributing to resistance due to a gyroscopic effect. also, known as 2A or two hands looping freestyle.
Photos from as early as the late 1950s show early yo-yo demonstrators performing very basic Triple A tricks, such as a Sleeper with one hand, and a Trapeze with the other. While Triple A as a concept has existed for many years, it was not until the debut of Velvet Rolls that development began on what is currently considered Triple A.
When the yo-yo is first released, the gravity (and the throw) give it translational kinetic energy and necessarily, since the string must unwind, much of this energy is converted into rotational kinetic energy establishing the free move of the yo-yo, and causing it to spin rapidly. As the yo-yo unwinds downwards, it also converts potential energy from gravity to both translational and rotational kinetic energy. Because the yo-yo has significant rotational inertia, it can store enough energy in its rotation to overcome gravity all the way back up to the hand.
