Oooh - this is a nice video. The idea is that when you heat or cool something, it doesn't change state automatically. There needs to be a bit of dirt or something for the new state to start on. So here they've got a bottle of clean water and it's cooled quite a bit below room temperature but hasn't frozen because it was clean. Then they shake it, which introduces an irregularity so it freezes.
You can get the same with boiling water, especially if it's in a microwave in a very clean cup. You can heat the water to 120C or so without it boiling. But then, if you let it touch your lips, the lips provide an uneven surface, so it all boils then and you end up with a faceful of steam, which is really not nice. Don't try that at home.
And I referred to that in an essay I wrote last week on the Reformation.