This launcher will fire tennis balls as long as it has electricity and a steady supply of balls. To do this, it uses air pressure.

When you turn the launcher on, an electric fan pulls air from the outside into the canister. The air flows through a piece of foam and a protective screen, which protect the fan motor from debris. Without any balls in the machine, the air just flows upward through the hopper and out through the barrel.

But when you add tennis balls to the hopper, things get interesting. An electric motor turns a rotor inside the hopper. As the rotor turns, it rolls the balls through a hole one at a time. Each ball falls into the plastic tube that runs through the canister. The tube connects the hopper to the barrel, and a gap in the middle of the tube allows air from the chamber to flow into it.

The ball rolls through the tube until it reaches a soft, rubbery ring called a detent. The detent holds the ball in place, so the ball seals off the end of the tube closest to the barrel. This causes the air from the canister to flow up through the hopper, forcing a plastic flap to cover the entrance to the tube. Now, with the tube blocked at both ends, the air pressure inside starts to build.

When the air pressure behind the ball overcomes the resistance from the detent, the ball flies down the barrel and is launched into the air. When the ball is gone and the pressure is eliminated in the tube, the flap at the entrance falls back down. Another ball falls into the tube, and the process starts again.

This pneumatic design isn't limited to launching tennis balls. T-shirt and confetti launchers at sporting events use the same concept. Pneumatic air cannons can also create special effects -- like explosions and flying debris -- for movies and TV.