Conservation of momentum

In any collision or interaction where no outside force gets involved, the total momentum of everything involved is exactly the same afterwards as it was before. It doesn't matter whether the objects bounce apart or crumple together — momentum is conserved either way.

What isn't guaranteed is kinetic energy. In a perfectly elastic collision, like two billiard balls colliding, both momentum and kinetic energy are conserved. In an inelastic collision, like a car crash where the vehicles stick together, momentum is still conserved, but a lot of kinetic energy is converted into heat, sound, and the work of crumpling metal.

A useful way to picture "no outside force": a system of exploding fireworks has momentum zero before the explosion and momentum zero after, because every fragment flying off in one direction is balanced by other fragments flying off in a way that cancels it out — the explosion is entirely internal to the system.