Why do balls bounce?
Short answer
When a ball hits the floor, it briefly deforms and tries to return to its original shape. The ball pushes on the floor, and the floor pushes back, sending the ball upward.
How it works
As a ball falls, gravitational potential energy becomes kinetic energy, the energy of motion. When the ball hits the floor, it changes shape and temporarily stores part of that moving energy inside its material.
As the ball returns to shape, that stored energy moves the ball again. Not all of it becomes upward motion: some becomes sound, heat, and friction. That is why the bounce usually reaches a lower height than the drop.
Key point
A ball that bounces well is made from a material that returns more of the deformation energy to motion. The ball, the floor, the temperature, and the drop height all affect how it rebounds.