It all comes down to hair. The hair on a new tennis ball tends to be smoothed flat, while a ball that's been knocked around a bit will be more fluffy. Tennis players may check three balls or more before serving so that they can select one smooth ball and one fluffy ball.

The smooth ball is used for the first serve. Because the hairs are flattened down, the ball travels faster than an older ball, which should make it harder to return. But the gain in speed comes at a cost. "The benefit is counteracted by less accuracy because you get less grip on the ball when you hit it," says Jan Magnus, of Tilburg University in the Netherlands. Should the first serve go astray, the player will use the fluffier ball for their second serve. Although these move slower, they are easier to control and so the player is less likely to concede a double fault.

Magnus and his colleague Franc Klaassen, of Amsterdam University, have analysed 100,000 points played at Wimbledon between 1992 and 1995. Their latest study looked at how effective serves are. They found that even top professionals often have a bad serving strategy.

"You can't make your first service too easy, because even though it'll go in every time, it'll be returned too easily. But equally, you see people using an enormous first service that almost never goes in. You have to find the optimum in the middle."

The judgment is complicated by having two serves. Magnus found that if a player lost form and started missing a lot of first serves, they often over-compensated by making the second serve too easy to return.

"This is the most common error," says Magnus. "They are too afraid of double faults, but double faults are not a bad thing. There's a big misunderstanding about that." Players who never concede double faults are not pushing themselves enough. "If you play to your limit, you will occasionally go over the line and get a double fault. But if you never go over it, you're too far away from it," he says.