Here's why:

* Velocity is a measure of both speed and direction.

* Acceleration describes how quickly that velocity changes.

This change in velocity can involve:

* Changing speed: If a car speeds up or slows down, its velocity is changing, and therefore it's accelerating.

* Changing direction: Even if a car maintains a constant speed, if it turns, its direction changes, meaning its velocity changes, and it's accelerating.

So, acceleration is not just about speeding up; it's about any change in velocity, including slowing down (deceleration) and changing direction.