Here's why:

* Acceleration is the rate of change of velocity.

* Zero acceleration means the velocity isn't changing.

* Therefore, if the velocity is not changing, it must be constant.

Examples:

* An object moving at a steady 10 meters per second in a straight line has zero acceleration.

* A car parked in a driveway is not moving, so its velocity and acceleration are both zero.

Let me know if you have any other questions!