* Acceleration is the rate of change of velocity.

* Constant velocity means the velocity is not changing.

If the velocity isn't changing, then the acceleration must be zero.

Think of it this way:

* Acceleration is like pushing the gas pedal in a car. You're speeding up (positive acceleration) or slowing down (negative acceleration).

* Constant velocity is like driving at a steady speed on a straight road. You're neither speeding up nor slowing down.