Here's why:

* Acceleration is the rate of change of velocity. Velocity includes both speed and direction.

* Therefore, acceleration can change both speed and direction.

Examples:

* A car speeding up: This changes speed, so it's accelerating.

* A car turning a corner at constant speed: This changes direction, so it's accelerating.

* A ball thrown upwards: This changes both speed (it slows down) and direction (it changes from upwards to downwards), so it's accelerating.

In summary, acceleration involves a change in velocity, which can be a change in speed, a change in direction, or both.