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 includes both speed (how fast the object is moving) and direction.

Therefore, acceleration can occur in two ways:

1. Change in speed: If the object is speeding up or slowing down, its acceleration is non-zero.

2. Change in direction: Even if the object maintains a constant speed, if its direction changes, it is accelerating. For example, a car moving in a circle at a constant speed is still accelerating because its direction is constantly changing.