Get an unopened bottle of water, most preferably purified or distilled water.

If you live where the temperature outside is below freezing put the bottle of water outside and for about 3 hours, otherwise just put the bottle of water in your freezer for about two and a half hours (the time varies according to your freezer's size and output and the temperature you set it to.)
Carefully take the bottle of water. If it is frozen, then that is a sign of failure.
Shake the bottle a few times until you see some parts of the water beginning to freeze.
Congratulations, you have just supercooled water!
ALTERNATIVE
Pour 20 ml/1.25 tbsp of purified or distilled water into a glass.
Put the glass in a bowl taller than the glass.
Fill the bowl with ice so the glass is surrounded by ice. (NEVER actually put ice in the glass.)
Sprinkle two tablespoons of salt over the ice. (again NEVER sprinkle salt in the glass, we want the water to be as pure as possible.)
Optional. Put a clean thermometer in the glass, or take a leaf out of the Mythbuster's superheated water book and put a bottle of normal tap water in with the purified water. When the tap water freezes, the pure water's been supercooled.
Wait about 15 minutes or when the thermometer shows the water is below freezing. If the water in the glass is frozen you failed.
Drop a piece of ice in the glass, watch what happens.
Congratulations, you have just supercooled water!

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