Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A stress ball is a malleable toy, usually not more than 7 cm in diameter filled with a soft material used for stress and muscle tension in your 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.