Here's why:

* Gravity: The primary force acting on the ball is gravity, which constantly pulls it downwards.

* Upward Motion: When the ball is thrown upwards, it has an initial upward velocity. Gravity gradually slows this velocity down.

* Top of the Path: At the peak of its trajectory, the ball momentarily stops moving upwards. It's not yet falling back down, so its upward velocity has become zero.

* Instantaneous: "Instantaneous" means at that precise moment in time.

Important Note: While the speed is zero at the top, the ball's *acceleration* is not zero. Gravity is still acting on it, causing it to accelerate downwards, preparing for its descent.