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, any change in either the speed or the direction of an object results in a change in its velocity, and thus, an acceleration.

Examples:

* A car speeding up on a straight road is accelerating because its speed is increasing.

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

* A ball thrown straight up in the air is accelerating due to gravity, slowing down as it goes up and speeding up as it comes down.

Let me know if you would like to know more about acceleration or velocity!