Several devices are available to produce vortex rings filled with smoke. The small plastic models have their own smoke sources included and can blow out a candle when close up.
The next larger size, the black can, has considerably greater range, but needs the smoke generator, as does the trash can version.
Questions:
Can SQUARE rings be made?
What does the speed depend on?
A dolphin playing with bubble rings can be found on YouTube at: http://www.youtube.com/watch?v=TMCf7SNUb-Q
as of 3/2009
