If you can't afford a lot of new balls, used ones are fine for informal practice. There's a reason why professional tournaments rotate the balls every 9 games or so. New balls are more durable and feel better in your hand. However, if you plan to play in public venues or at high levels of competition, it's best not to use old balls because they won't conform to the required standard of quality and durability. The material used in the construction of tennis balls is natural rubber. It's been around for many years and remains popular today because it feels good when you hit a ball with it and also provides ample freedom of movement after being stretched during play. However, natural rubber balls do wear out over time and need replacing about every 300-500 swings. This is because the rubber itself is organic and grows harder as it ages. While natural rubber balls are still used by some professionals, most use something else instead. For example, one of the most popular materials used now is synthetic rubber. It works just like natural rubber but lasts much longer and doesn't break down under pressure like its natural counterpart does. Some manufacturers even produce balls made from different layers of synthetic material that remain soft and flexible even after being stretched several times beyond their original size.