A hockey puck is frozen a few hours before games to reduce bouncing during play. Cold temperatures make the vulcanized-rubber puck harder and less lively, helping it stay flatter and behave more predictably on the ice.
A standard ice hockey puck is a black disk about 3 inches in diameter and 1 inch thick. It is made from vulcanized rubber, a material chosen for durability and controlled movement. If the puck is warm, the rubber becomes more flexible and can bounce or wobble more readily after contact with a stick, the boards, or another player's skate.
Professional leagues keep game pucks in freezers and bring them out shortly before use. International Ice Hockey Federation guidelines specify storage between approximately -10 and -12 degrees Celsius. Officials typically replace a puck during a stoppage if it becomes damaged or otherwise unsuitable.
Freezing does not make the puck completely unable to bounce; hard shots and awkward impacts can still send it airborne. The purpose is consistency, not simply speed. The cold puck also resists becoming soft and sticky against the ice. Early hockey pucks varied in material and shape, but the modern frozen rubber disk is now standard in organized ice hockey.