What Causes a Ball to Bounce?

Gravity and air pressure are what cause a ball to bounce. When you throw a ball down gravity is what causes it to go down. Then it will hit the ground and that compresses the air inside the ball, this causes the air to then expand and bounce the ball back into the air. Then gravity keeps the ball from floating away and pushes it back down to the ground again. This process repeats itself over and over until the ball eventually stops bouncing.