Here's why:

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

* Speed is the rate at which an object moves, a scalar quantity.

So, even if an object maintains a constant speed, if its direction changes, its velocity changes.

Examples:

* A car traveling in a straight line at a constant speed: Its velocity is constant.

* A car turning a corner at a constant speed: Its velocity changes because its direction changes.

* A car accelerating from rest: Its velocity changes because its speed increases.

* A car slowing down to a stop: Its velocity changes because its speed decreases.