My additional musing: maybe he used glycerol. Its hydrophilic, clear, thicker than water, and maybe it grabbed enough water to force crystals out of solution.
To clarify E, is this a video that was found from a reputable source, where the person is trying to engage the audience to think about a problem, if so, there are things about the video that we know that may be taken for granted, ie if it was geared to high school students, the material covered is more limited than that to university level.
If the video is found online, the source may be less than reputable and the person in it may be concealing certain facts that would clarify things, ie if I made a video about how to make milk glow in the dark, didn't say I added luminol to the solution, then challanged anyone to tell me the principles at work.