Here's why:

* Gravity is always acting: Even though the ball momentarily stops at its highest point, gravity is constantly pulling it downwards.

* Acceleration due to gravity: The acceleration due to gravity (g) is approximately 9.8 m/s² near the Earth's surface. This means that for every second an object is in freefall, its velocity increases by 9.8 m/s downwards.

Important Note: While the velocity of the ball is zero at its highest point, its acceleration is not. Acceleration is the rate of change of velocity, and the velocity is still changing even if it is momentarily zero.