Here's why:

* Change in Speed: This is the most intuitive form of acceleration. If you are speeding up or slowing down, you are accelerating.

* Change in Direction: Even if your speed remains constant, a change in direction is considered acceleration. Imagine driving a car around a roundabout at a steady speed. You are still accelerating because your direction is constantly changing.

In summary, acceleration is the rate of change of velocity, and velocity encompasses both speed and direction.