Here's why:

* Gravity: The force of gravity is constantly acting on the object, slowing it down as it moves upwards.

* Zero Vertical Velocity: At the peak of its trajectory, the object has completely lost its upward velocity. For a brief moment, it stops moving vertically before gravity starts pulling it back down.

Important Note: While the object's vertical velocity is 0 at the top, it might still have a horizontal velocity if it was launched at an angle. This is why projectiles follow a curved path!