Any object being acted on by gravity has constant acceleration,
and it's the same number for every object.
On Earth, the acceleration of gravity is 9.8 m/s² downward.
That means that an object moving down moves 9.8 m/s faster every
second, and an object moving up moves 9.8 m/s slower every second
(and eventually runs out of upward gas and starts moving downward).
At the top of its path, the rock's velocity changes from upward to downward.
If you change from upward velocity to downward velocity, the change
has to be smooth ... There's no way to jump from upward to downward
without going through every value in between. There has to be a split
second when you're not moving upward OR downward. At that split
second, your velocity is zero.
