I volunteer annually at Escondido Elementary School Science night for volcano & earthquake themed science demos. The following photos show one of my all time favorites - vortex cannon (I did not design the demo), which demonstrates the physics behind the "smoke rings" observed during volcanic unrests on top of the eruption vents. The smoke rings form when air flow speed at the center of the bucket opening significantly exceeds that at the edge of the opening (as would be expected due to viscous drag from the bucket walls). The radially outward torque resulting from slower moving air in the peripheral of the air jet causes the flow to wrap onto itself, forming forward propagating rings, which are made visible by the entrained stage fog.