Here's why:

* Motion is relative: Motion is not absolute; it's always defined relative to a reference point.

* Reference point: This is a fixed location that we use to determine if something is moving or not.

* Changing distance: If the object's distance from the reference point remains constant, it's considered at rest, even if it's moving relative to another object.

Example:

* You're sitting in a car that's moving at a constant speed. You are at rest relative to the car, but you are in motion relative to a tree by the roadside.