You don't actually hit them better. When you hit a dead ball it sounds like you hit it much louder, but a new ball is always ideal for hitting.
New tennis balls are considered fresh and not "flat". Old tennis balls exposed to play and air become flat and do not perform normally in a match.
The centers of tennis balls are rubber and rubber hardens with age.
Practice and hit balls at the driving range.
Give them to someone with a dog.
New tennis balls bounce higher than old tennis balls because they have more elasticity and air pressure which allows them to compress and decompress more easily upon impact with the ground. As tennis balls are used, the felt covering wears out and the air inside the ball escapes, leading to less bounce.
They improved them a lot! If your question would be more specific, it would be easier to answer. The tennis balls used to be wads of hair or wool wrapped in stuff, but now they are more rubbery for bounce. I don't know exactly how.
The 'people who fetch the tennis balls' are called ball boys and ball girls. The ball boys and ball girls are usually under 18 years old, with some being as young as 10 or 12 and others being as old as 16.
A new tennis ball will bounce higher. Old tennis balls are considered "dead."
Before television was invented, tennis balls were white. They would have remained this way if television had not been broadcast on live TV This was because people couldn't see the white tennis balls flying across the screen. So, they were changed to neon green/yellow so that viewers could see them.
Hit a few balls, try it out.
Tennis balls are very useful for more than just tennis. Old people put them on their walkers, and people throw them to their dogs. But tennis players use them for, well, tennis. I think the biggest market would be rackets and re-stringing them.
Tennis balls are geared for different court surfaces. There are tennis balls specifically designed for clay, hard court, and grass court surfaces. Check on the outside of the tennis ball can for the type of court surfaces you should use them on. For example, some tennis balls are specificially made for clay court surfaces. Some tennis balls work on hard court surfaces both indoors and outdoors. Check out your local sports store for many different brands. You also want to make sure that the balls you are playing with are "flat". Tennis balls lose their air pressure after you play with them. You can keep them in a special pressurized tennis ball can, but these are not used very much anymore. Make sure to open a new can of tennis balls when you hit the tennis course for a match. In fact, this a rule in tennis tournaments and leagues. You can squeeze the tennis ball to determine if it is flat. If you can easily squeeze it then it is most likely flat. Or you can bounce the ball to tell. Old tennis balls should be thrown away if they have lost most of their fuzz as well.