The requirements for smoke rings include having smoke in the respiratory tract, a certain amount of practice and lips that are set in an almost perfectly circular shape. The trick is to breathe out relatively small amounts of air in order to create circular swirls. The air flows from the throat through the mouth and is slowed down slightly at the sides when it reaches the lips, although it continues to travel unhindered through the centre of the mouth. Ideally, this situation creates a ring of turbulence.The air in this ring rotates relatively quickly and so traps smoke particles, thereby making its shape visible. If this happens in relatively drought-free surroundings, the smoke rings may last several minutes.
The Earth can also produce smoke rings-which are occasionally seen when a volcano erupts-and so can jet aircraft and guns when the conditions are right.