Here's why:

* Velocity is a measure of both speed and direction. Constant velocity means moving at the same speed in the same direction.

* Acceleration is the rate of change of velocity. If velocity is constant, it's not changing, so there's no acceleration.

It's possible to have a constant speed but not a constant velocity, which would result in acceleration.

Example:

Imagine a car driving around a circular track at a constant speed. Even though the car's speed is constant, its velocity is changing because its direction is constantly changing. This change in direction means the car is accelerating, even though its speed is constant.

Key takeaway: Acceleration requires a change in velocity, either in speed or direction.