Here's why:

* Velocity is a vector quantity, meaning it has both magnitude (speed) and direction.

* Acceleration is the rate of change of velocity over time.

Therefore, acceleration can occur in the following ways:

* Increasing speed: The object moves faster in the same direction.

* Decreasing speed: The object moves slower in the same direction (this is also called deceleration or retardation).

* Changing direction: The object maintains its speed but alters its direction of motion.

In summary: Acceleration is any change in an object's velocity, whether it's a change in speed, direction, or both.