They are formed out of two hard plastic marbles, each about two inches in diameter, attached to a ring with 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. After a little 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 several children got injured: besides being fairly heavy and fast-moving, the spheres, being made of hard acrylic plastic, could shatter upon striking each other, presenting a safety hazard.
There is a recurring urban legend that the clacker spheres were made of glass, although no one has yet produced an actual glass set. The confusion may have arisen because many sets were made of transparent colored acrylic which resembled glass.
The toys enjoyed a brief renewal of popularity in the 1990's when clackers consisting of a handle and plastic triangles with the plastic marbles at the end giving weight to the free moving toy. It was played the same way and were always in bright neon colors.