Here's why:

* Gravity: The only force acting on the ball once it's thrown is gravity, which pulls it downwards.

* At the highest point: As the ball travels upwards, gravity slows it down until it momentarily stops at its highest point.

* Change in direction: At that instant, the ball's velocity changes direction from upward to downward. Since it's changing direction, it has to pass through a point where the velocity is zero.