Here's why:

* Gravity is a force of attraction: Every object with mass exerts a gravitational pull on every other object with mass.

* Earth's mass is huge: Because Earth is incredibly massive, it has a strong gravitational pull.

* Gravity acts on the ball: This means the ball is constantly being pulled towards the center of the Earth.

This is why a ball thrown upwards eventually slows down, stops, and falls back down to the ground.