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 weight 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 onto 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.