A tennis ball is pressurised so that it bounces predictably on hard court surfaces.

The longer version

Air inside the hollow rubber core pushes outward at well above atmospheric pressure, and that internal pressure is what makes the ball spring back when it hits the ground. The felt cover adds drag and grips the strings briefly, but most of the bounce energy comes from the compressed gas rather than the rubber itself. As pressure leaks through the seams over weeks, the ball gradually goes dead.

Rules specify a rebound height when a ball is dropped from a fixed height onto a hard surface, which measures pressure without opening it. Pressureless balls exist and last longer, but they feel heavier and sound different off the strings. Storage in sealed containers slows the leak, which is why players who care about feel keep track of how long a can has been open.

Where this entry sits

Facts here look at the rules, equipment and odd histories behind familiar sports and games, from balls and rackets to cards, boards and playing fields. Expect small details about scoring quirks, stadium design and the long story of the Olympic Games. The full list sits on the Sports and Games page, where every entry is listed in order.