Here's why:

* Simple Harmonic Motion: Vibrations often follow a pattern called simple harmonic motion. This motion is characterized by a restoring force that is proportional to the displacement from equilibrium.

* Turning Points: The points of maximum displacement are called turning points. At these points, the object momentarily stops before changing direction.

* Velocity and Displacement: Velocity is the rate of change of displacement. Since the displacement is momentarily not changing at the turning points, the velocity is zero.

Think of a swinging pendulum:

* When the pendulum reaches the highest point of its swing (maximum displacement), it momentarily stops before swinging back down. At this point, its velocity is zero.