From Michael Colombo … Dripping Water Turned into Standing Waves:
Facebook user Brusspup performed a simple, but interesting, experiment. Passing a tube of water across the front of a loudspeaker yields a standing wave when tuned to a 24 hz sound wave. Then when switched to 23 hz, it appears that the water droplets are actually traveling backwards through the tube they just came out of. A camera recording at a standard 24 fps is playing tricks with the eye when the frequency of the sine wave is very similar.