Acceleration is the rate of change of velocity over time.

Here's why:

* Velocity includes both speed (how fast an object is moving) and direction.

* Acceleration measures how quickly an object's velocity changes. This change can be in speed, direction, or both.

Examples:

* A car speeding up: The car's velocity increases, meaning it has acceleration.

* A car slowing down: The car's velocity decreases, meaning it has acceleration (in the opposite direction of its motion).

* A car turning a corner: Even if the car maintains a constant speed, its direction changes, which means its velocity changes, resulting in acceleration.

In short, acceleration is about how the velocity of an object changes, not just its direction.