system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are tennis players looking for when they request 3-4 tennis balls and immediately toss 1-2 of them back?

(also, go Djokovic! :P)

Edit: Thank you all for a good discussion/explanation! I hope you are all ready for the Wimbledon final! Djokovic v Federer. :)
gpt
To explain this to a 5 year old, essentially they are testing (for lack of a better word) the bounce of the ball. Because they hit the ball so hard they wear and tear causing the balls to bounce in different places making the game difficult. It also loses the energy that is put into the ball when hitting it. For example, when you kick a flat football it will stop much quicker than if you were to kick a fully inflated football.