Here's a breakdown:

1. Gravity: When the ball falls, gravity pulls it down.

2. Impact: When the ball hits the ground, it stops moving downward momentarily.

3. Compression: The impact causes the ball to deform slightly, compressing its internal structure. This compression stores energy.

4. Elasticity: Most balls are made of elastic materials, meaning they can regain their original shape. As the ball springs back from the compressed state, it releases the stored energy.

5. Rebound: This release of energy pushes the ball back upwards, against the force of gravity.

In simpler terms: The ground basically "pushes back" on the ball harder than the ball pushes down on the ground, causing it to bounce upwards.