Here's why:

* Velocity is a measure of how fast an object is moving and in what direction. It's a combination of speed and direction.

* Acceleration is the rate at which velocity changes over time.

Think of it like this:

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

* If you press the gas pedal, you're increasing your speed, and therefore your velocity is changing. This change in velocity over time is what we call acceleration.

* If you brake, you're decreasing your speed, meaning your velocity is changing again, but this time with negative acceleration (deceleration).

In summary:

* Acceleration is the change in velocity over time.

* A positive acceleration means the velocity is increasing.

* A negative acceleration (deceleration) means the velocity is decreasing.

* Zero acceleration means the velocity is constant.