A stress ball is a malleable toy, usually not more than 7 cm in diameter. It is squeezed in the hand and manipulated by the fingers, ostensibly to either help relieve stress and muscle tension or to exercise the muscles of the hand. There are many types of stress balls. Many are a closed-cell polyurethane foam rubber. This type of stress ball is 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.
The Daily Omnivore 