If you only watch one optical illusion today in which a stream of water appears to have droplets freeze in mid-air or inch their way backwards back into the tube from whence they came, make it this one.
The effect is caused by syncing up a camera with the vibrations of the water. The speaker behind the stream of water is generating a 24 hz sine wave, and the camera is filming at 24 fps, which causes the water to appear frozen. Or, you can adjust the hz rating of the wave and move the stream very slowly forwards (at 25 hz) or, much crazier for our purposes, very slowly backwards, at 23 hz. Check out the video: