Making smoke rings should be done after the tornado demonstration and is an illustration that turbulence and other small vortices are stable only if the end of the axis of a vortex either rests on a surface such as the Earth's surface or ends on itself as in a smoke ring. Other vortices of tornado scales or smaller do not last long and decay rapidly. Larger vortices such as the Taylor vortices which extend horizontally for 100 to 500 miles are relatively stable but even the axes of these may end on solid surfaces.
Smoke rings can be demonstrated in the classroom by cutting a hole around half an inch in diameter in the bottom of a plastic margarine dish or can. Glue two small pads of blotter paper inside and add carefully add a drop of dilute HCL (Muriatic acid) to one and a drop of household ammonia to the other. Replace the plastic lid. Tap gently on the lid and a smoke ring will come out of the hole. These persist for a considerable length of time and are generally stable even in a light wind.