Here's why:

* Speed: Acceleration happens when an object speeds up or slows down.

* Direction: Acceleration also occurs when an object changes its direction, even if its speed remains constant.

Examples:

* A car speeding up on the highway is accelerating.

* A car slowing down to a stop is accelerating (in the negative direction).

* A car turning a corner at a constant speed is also accelerating because its direction is changing.

Important Note: Acceleration is a vector quantity, meaning it has both magnitude (how much) and direction.