Tennis balls are made up of a hollow rubber core and a wool or nylon shell known as the nap.
In tournaments they must be white or yellow in colour.
The rubber core is usually filled with pressurised air in order to make the ball bounce.
The ball must be between 2.5 inches (6.25 cms) and 2.63 inches (6.57cms) in diameter and must weigh at least 56 grammes, but no more than 59.4 grammes.
The rules of tennis say that the ball must bounce between 53 and 58 inches after being dropped on to concrete from a height of 100 inches.
That is why balls are changed during tournaments - they begin to lose their bounce after about three hours of play.
At Wimbledon they are changed after the warm-up and the first seven games, and after that, every nine games.
Studies have shown the fluffier a ball gets, the slower it flies, which is why you often see top players check several balls to find the least fluffy before serving.
When not in use, tennis balls should be stored in the can they came in. This won't prevent them from losing pressure, but can slow it down.