Playing With Fire in Slow Motion
In this short video, photographer Nick Moore shows us what happens to flame when it's exposed to a speaker vibrating at 60 Hz. Although you can't see the effect easily at regular speed, when the video is shown in slow motion, there is a clear pulsing action as the speaker pushes and pulls the air, causing flames to dance rhythmically.
Stick around at least until the 0:45 mark, when you start to see the slow-mo fire effects.
See also: Moore's experiment with cymatics, Mercury + Sound = Kooky Mad Science Party.