This is a hands-on demonstration. Try this with your friends, and be prepared to explain the physics of the demonstration to them. When the can is dropped, it free falls along with the water and the ball. They become weightless (in the accelerating reference frame). In this weightless environment, the wet ball is pulled underwater by surface tension. This shows that the surface tension of water is quite significant, though its effects are normally masked. When the can hits the ground, the submerged ball suddenly feels the buoyant force of all the water it has displaced. Also, the water that slams into the bottom of the can rebounds, and this causes a compression wave. The net effect is that the ping-pong ball is shot upward very rapidly. Wear your safety glasses!