Sound & Water Experiment (Illusion)

The explanation is as follows:
The speaker creates vibrations in the tube, and accordingly in the flow of water, at the set frequency. The camera is recording at 24 fps (frames per second). So if the sound frequency is set at 24Hz, the camera will record each frame at the exact same point in the wave as the previous one, making the water seem still. What is actually happening is that since the vibrations cause very similar patterns in the water flow every time, then each frame will look very similar to the previous one. If the frequency is set slightly lower at 23Hz, the camera catches a slightly earlier frame of these very similar patterns each time, so it looks as if the water is flowing upwards slowly. At the slightly higher 25Hz, the camera catches a slightly later frame of the same pattern each time, so it looks as if the water is flowing downwards slowly.