Here's why:

* Air Resistance: As a ball moves through the air, it collides with air molecules. These collisions create a force that opposes the ball's motion, slowing it down. This is known as air resistance or drag.

* Rolling Friction: If the ball is rolling on a surface, there is friction between the ball and the surface. This friction creates a force that opposes the ball's motion, slowing it down.

* Other Forces: In some situations, other forces might contribute to slowing down a ball, such as:

* Gravity: If the ball is moving upwards, gravity will pull it down, slowing it down.

* Water Resistance: If the ball is moving through water, water resistance will slow it down.

In summary, friction is the primary force that slows down a ball when it moves.