Here's why:

* Velocity is a measure of how fast something is moving and in what direction.

* Acceleration is a measure of how quickly the velocity is changing.

Think of it this way:

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

* If you step on the gas pedal, you are increasing your velocity, and your acceleration is positive.

* If you brake, you are decreasing your velocity, and your acceleration is negative (also known as deceleration).

Acceleration is a vector quantity, meaning it has both magnitude (how much) and direction.