Relatively pure water cooled slowly below the freezing point may remain liquid (supercooled). It may then crystallize into ice quickly by some catalyst: here, strong vibrations, and seeding by an ice crystal.

And no -- for those who have seen sodium acetate and don't understand supercooled liquids -- this is NOT sodium acetate; it is just pure drinking water. Don't even bother suggesting otherwise; this issue has been covered completely in the comments.

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