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Probably just water and a bit of sneaky supercooling

You lower the temperature to below freezing but without anything to crystallise off, then give it knock or introduce something for it to crystallise off.

You can do it with beer too

It is a supersaturated solution, you heat up the liquid while dissolving loads of salt it is (the hotter it is the more you can dissolve) then you cool it.

Then as you say a slight knock or the introduction of a crystal pattern will act as a nucleation site for the crystallisation.