A stress ball is a malleable toy, usually not more than 7 cm in diameter, which is squeezed in the hand and manipulated by the fingers, ostensibly to relieve stress and muscle tension or to exercise the muscles of the hand.
Despite the name, many stress balls are not spherical. Some are molded in amusing shapes, and pad- or transfer-printed with corporate logos. They are presented to employees and clients of companies as promotional gifts. Stress balls are the third most popular promotional gift in the United Kingdom. Because of the many non-spherical shapes now available, stress balls are generically known as stress relievers.
There are several different types of stress balls that originate from many different countries. The most common type of stress ball in America is the “bean bag” type, commonly known as a “Hacky Sack”. The stress ball that is most common in Australia is the foam type, this type prevents stress through resistance from squeezing the ball. The third type of stress ball is the Chinese form known as the Baoding ball. These are not like the others as these are not squeezable, they are solid, they usually come in pairs so you can roll them together to make a soothing sound and a smooth sensation feeling in your hands.
Some stress relievers are made from closed-cell polyurethane foam rubber. These are made by injecting the liquid components of the foam into a mold. The resulting chemical reaction creates carbon dioxide bubbles as a byproduct, which in turn creates the foam.
Stress balls, especially those used in physical therapy, can also contain gel of different densities inside a rubber or cloth skin. Another type uses a thin rubber membrane surrounding a fine powder. The latter type can be made at home by filling a balloon with baking soda. Some balls similar to a footbag are marketed and used as stress balls.