Here's why:

* Velocity is a measure of both speed and direction.

* Acceleration describes how the velocity of an object changes. This could be a change in speed, a change in direction, or both.

Think of it like this:

* If you're driving at a constant speed in a straight line, your velocity is constant, and your acceleration is zero.

* If you speed up, your velocity is increasing, and you have positive acceleration.

* If you slow down, your velocity is decreasing, and you have negative acceleration (also called deceleration).

* If you turn a corner at a constant speed, your velocity is changing because your direction is changing, and you still have acceleration.