Watch a server at a tournament and you'll notice a small ritual: they hold up two or three balls, spin them, reject one, maybe reject another, then settle. It looks like superstition. It isn't. A tennis ball is a small pressurised engineering project, and by the time it reaches your hand it has already started to change.

It's a balloon with a conscience

A standard pressurised tennis ball is a hollow rubber core filled with air (or, in some balls, nitrogen) at a pressure slightly above the surrounding atmosphere. That internal pressure is what makes the ball lively: when it hits the strings, the core compresses and springs back, returning energy to the shot. The felt — a mix of wool and nylon, usually with a bit of cotton — is glued on top. It does two jobs: it grips the strings so the ball can be spun, and it protects the rubber underneath.

The key point: a pressurised ball starts losing pressure the moment it's sealed, and faster once the can is opened. A ball left out overnight plays noticeably deader than one fresh from the can. This is why balls come in a sealed, pressurised can — the can is effectively a second balloon keeping the first one honest. It's also why the old trick of pressing the can lid to check the seal exists at all.

Fluff is not a flaw — at first

New balls are fluffy. The fibres stand up, and for the first few games that fresh felt grabs the strings hard: more friction, more spin, a slightly higher bounce off the court. Players often talk about new balls being "faster" — they're not necessarily travelling quicker, but they skid and kick differently, and the extra grip changes how the ball reacts to a heavy topspin shot.

Then the fluff knocks down. The fibres mat, the surface gets smoother, the ball gets a little smaller and lighter as felt wears off, and the bounce settles. So a match has a texture to it: the lively, grabby opening games, the settled middle, and the flatter, deader end. Part of the skill of playing a long match is noticing which phase you're in and adjusting.

Why three balls, not one

Here's the practical bit. When a server examines several balls, they're doing a quick quality-control pass. A ball with a visible flat spot, a seam lifting, or felt worn through to the rubber will fly unpredictably — and on a serve, where you're trying to hit a small target at high pace, unpredictability is expensive. Rejecting the odd one out is a tiny act of risk management.

There's a tactical layer too. Some players prefer the freshest, fluffiest ball for a first serve, because the extra grip helps the kick or slice bite. Others like a slightly worn ball for a flat serve, or for the second serve where a bit more control matters. And servers often glance at the balls the receiver is holding, because in most formats the server serves with the same set — knowing whether the receiver has the new one or the old one is useful information.

The science in your own bag

You don't need a laboratory to use this. Balls you've had in your bag for a month are not the same balls you bought. If you're practising serves, use the freshest ones for the serves that matter and save the tired ones for feeding or for drills where you want the ball to sit up. If a ball feels soft or looks bald, retire it — a dead ball teaches your body the wrong timing. And on match day, if you're handed the choice, take the moment to look. It's not a ritual. It's a read.

Take it to court

• Rotate your match balls. Keep a fresh set for matches and serves, and demote older balls to practice feeding. A ball's pressure and felt both degrade, and your technique adapts to whatever you hit.
• Inspect before you serve. Spin the ball in your hand, check for flat spots and worn felt, and reject the outlier. It's a two-second habit that removes a variable.
• Notice the phase of the match. Fresh balls grab and kick; worn balls sit down. Adjust your targets and your margins as the set wears on, rather than blaming your swing.