Here's why:

* Velocity is a measure of both speed and direction.

* Acceleration is the rate at which velocity changes.

This means acceleration can occur in two ways:

1. Change in speed: If an object speeds up or slows down, its velocity changes, and therefore it is accelerating.

2. Change in direction: Even if an object maintains a constant speed, if it changes direction, its velocity changes, and it is accelerating.

Examples:

* A car speeding up from a stoplight is accelerating.

* A car slowing down to a stop is also accelerating (in this case, decelerating).

* A car turning a corner at a constant speed is accelerating because its direction is changing.

Let me know if you'd like more examples or explanations!