Here's why:

* Acceleration is the rate of change of velocity. This means that acceleration describes how quickly the velocity of an object is changing.

* Velocity has both speed and direction. So, acceleration can cause a change in an object's speed, its direction, or both.

Here are some examples:

* A car speeding up: Its speed is increasing, so its velocity is changing, meaning it is accelerating.

* A car slowing down: Its speed is decreasing, so its velocity is changing, meaning it is accelerating (even though it's slowing down).

* A car turning: Its direction is changing, so its velocity is changing, meaning it is accelerating.

Let me know if you'd like more detail on any of these concepts!