Some rings to rule them all

When I was a fire-eater, I was also a fire-breather. On still days I could blow a bubble of fire and leave an ascending smoke ring. I would expand as it went up. The largest I ever blew was about 25 feet across. Once and once only I achieved the fire-breathers’ coup de grace – a smoke ring which ascended inside another smoke ring without the second fire bubble destroying the first ring. Them were the days!