Here's why:

* Acceleration is the rate of change of velocity.

* Velocity has both speed and direction.

* So, acceleration means the object's speed, direction, or both are changing.

Example:

* A car speeding up is accelerating. Its speed is increasing.

* A car slowing down is also accelerating. Its speed is decreasing.

* A car turning a corner is accelerating. Its direction is changing, even if its speed remains constant.