Here's why:

* Acceleration is a vector quantity: This means it has both magnitude (how fast the velocity is changing) and direction.

* Velocity is also a vector quantity: It has both speed (how fast an object is moving) and direction.

Therefore, if an object changes direction, even if its speed remains constant, its velocity changes, leading to acceleration.

Examples:

* A car turning a corner: The car's speed might be constant, but its direction changes, meaning its velocity changes, resulting in acceleration.

* A ball moving in a circle: The ball's speed might be constant, but it's constantly changing direction, resulting in acceleration towards the center of the circle (called centripetal acceleration).

In summary, acceleration is about the change in velocity, and velocity includes both speed and direction. So, any change in direction, even without a change in speed, results in acceleration.