NHL hockey pucks are frozen before games to prevent them from bouncing.

The National Hockey League (NHL) requires home teams to freeze pucks for at least four hours before a game to reach a temperature between 14 and 20 degrees Fahrenheit (-10 to -6 degrees Celsius). This process utilizes the physical properties of vulcanized rubber, which is a polymer that becomes significantly less elastic as its temperature drops. At room temperature, the rubber molecules are flexible and store energy upon impact, causing the puck to bounce unpredictably when it hits a stick or the boards.By cooling the pucks, the molecular motion within the rubber slows down, making the material stiffer and more rigid. This reduction in elasticity ensures that the puck absorbs less energy during collisions, keeping it 'dead' on the ice for better puck handling and passing accuracy. A study by the NHL found that a frozen puck is far more stable than one at room temperature, which can bounce like a tennis ball if hit hard enough.During a typical game, officials use approximately 25 to 30 pucks to ensure they stay cold. Pucks are kept in a portable cooler or a freezer located near the penalty box and are replaced every few minutes or during television timeouts. Since 2019, the NHL has even used 'thermocolor' pucks with ink that changes from purple to white when the puck warms up above freezing. This visual indicator tells officials exactly when it is time to swap the puck for a fresh, frozen one.