Here's why:

* Conservation of Momentum: The total momentum of a closed system (one where no external forces act) remains constant. This means the momentum *before* a collision is equal to the momentum *after* the collision.

* Collisions can be elastic or inelastic:

* Elastic collisions: Kinetic energy is conserved. Think of a perfectly bouncy ball hitting a wall.

* Inelastic collisions: Kinetic energy is not conserved. Think of a car crash, where some energy is lost as heat, sound, and deformation.

In summary: While momentum can be redistributed among the objects during a collision, the total momentum of the system remains the same.