Here's why:

* Momentum = Mass x Velocity

* Mass: A measure of how much matter an object contains.

* Velocity: A measure of how fast an object is moving in a specific direction.

* Objects at rest have zero velocity. Since their velocity is zero, their momentum is also zero, even if they have mass.

Examples:

* A stationary car has no momentum.

* A rock sitting on a table has no momentum.

* A book lying on a shelf has no momentum.

However, objects in motion do have momentum.

* A moving car has momentum.

* A thrown ball has momentum.

* A flowing river has momentum.

So, only objects that are in motion possess momentum.