Here's why:

* Going Up: When you throw the ball upwards, it has an initial upward velocity. Gravity acts on the ball, slowing it down until it momentarily comes to a stop at the highest point.

* At the Top: At the peak, the ball has stopped moving upwards and is about to start falling back down. This means its velocity is zero.

* Coming Down: As the ball falls back down, its velocity increases in the downward direction.

Think of it like this: Imagine you're throwing a ball straight up and it hits a ceiling. At the moment of impact, the ball's velocity is zero because it has stopped moving. The same principle applies at the peak of the trajectory.