The U.S. Open is underway in New York, and that means more tennis balls than you can shake a stick at. In case you didn't know, not all tennis balls come in cans. There are two types of tennis ball —pressureless and pressurized — and it’s the pressurized ones that come in cans. You might be thinking, "But wait, footballs, basketballs and volleyballs are pressurized and they don't come in cans." That's because other balls can be pumped up to the correct pressure with an air pump. Sealed cans allow manufacturers to control the pressure precisely inside the can, keeping tennis balls at their peak performance level for as long as possible. Pressurized tennis balls bounce because the air inside the ball is at a higher pressure than the outside atmosphere. As the ball hits your racket or the court, the pressurized air inside pushes against the rubber and helps it resist deformation and the ball bounces. Pressurized balls are sold in a pressured can to prevent the air from gradually leaking out through the porous rubber wall. They have a hollow rubber core that is not airtight, and once that rubber core is formed and covered with fuzz, that’s it — there’s no way to pump them up like you can a football or soccer ball. That distinctive whoosh when you open the can is the sound of the pressure being released from inside. Pressureless balls, however, are sold in a cardboard box. Since the air inside them is at regular atmospheric pressure, there’s no need to keep them in a pressurized can. So there you have it; now you know why tennis balls come in cans and footballs don't.
