Explain the Conservation of Momentum
In a collision, energy is not all time conserved, if not the collision is totally elastic (all stored energy is turned into kinetic energy, none is turned into heat, noise or damage), although all collisions have to conserve momentum if there is no net applied force.
∑ p before collision = ∑ p after collision
The easiest situation is the one dimensional case where you have balls running on a straight track.