In the video below, two carts are at rest on an air track. Gunpowder in a small chamber is placed between the two carts. When the gunpowder explodes, the carts are pushed apart, travel to the ends of the track, bounce, and return. When the carts collide with each other once again, they stick and remain motionless. The fact that the momentum of the system is conserved is thus graphically demonstrated.
However, everything does not always go smoothly in physics. Sometimes the professor uses too much gun powder!