Water famously becomes completely frozen at 32 degrees Fahrenheit (0 degrees Celsius). But when water is devoid of impurities, like in purified bottled water, the freezing process requires even colder temperatures.
So, if you place bottles of purified water in the cold air of a freezer and leave them a couple of hours, they'll still be liquid because pure water with no nuclei in it freezes at minus 43.6 degrees Fahrenheit (minus 42 degrees Celsius). It's now a supercooled liquid, which does indeed sound super cool.
Let's Make Some Instant Ice!
Ready to freeze water? Grab some water bottles and place them in your freezer. Make sure it's undisturbed for a few hours, getting it to that supercooled state. The exact freezing time? Typically, it takes about two-and-a-half to three hours.
Once the wait is over, remove the bottles with care. Then shake one or whack it on the table.
Anything can act as a nucleus at this point — air bubbles, a slight dent in the bottle. Any little change will be enough to cause homogenous nucleation. Once that disturbance is present, the uniform water molecules will freeze completely and so quickly that it looks instant.