DKO

You can cool water down below zero Celsius if the water is pure. The reason is that ice crystals need some impurity to form around.

 

It takes a little practice to get the time right, but if you put a bottle of mineral water in the freezer for a few hours you can get it below zero without freezing. Carefully get it out of the fridge and gently tap it. If you get this all right it will freeze solid in front of your eyes.

 

Try it also with tap water.

I've seen similar experiments done with bottles of Corona, after cooling for several hours you need only to tap the bottle on a counter top to introduce carbon dioxide bubbles which makes the freezing point drop so rapidly if freezes right before your eyes.