"Works just like a real supernova!"*
This demonstrates conservation of momentum. In this case, a large mass (all 4 balls) is dropped a short distance and achieves a small velocity at the time of impact. After the collision with the ground, a small mass (only the smallest ball) is shot upwards at a much larger velocity. Most effective if aimed at unruly students, cheeky lab managers.
*not really**
**but kinda sorta