is a term that refers to a vintage toy which was briefly popular in the early 1970s. Its name refers to the fact that its point was to have the two balls of which the toy consisted of clicking against each other.
They are formed out of two hard acrylic marbles, each about two inches in diameter, attached to a ring with a sturdy string. The player puts his or her finger in the ring, allowing the marbles (or balls) to hang below. Through a gentle up-and-down hand motion, the two balls swing apart and together, making the clacking noise that give the toy its name. With practice, it is possible to get the marbles swinging so that they knock together above the hand as well as below.
Clackers were eventually discontinued after some children were injured while playing with them. The spheres could shatter upon striking each other presenting a safety hazard.