Supercooling is what is happening when a liquid's temperature is brought below freezing point, but the liquid does not solidify into ice. The reason for this is the lack of a seed ice crystal or lack of particulate matter in the liquid. Because the formation of ice requires that there be something for it to start building its crystalline structure on, the lack of seed crystals or particles means no ice formation.

Hey kids! Here's an experiment you can try at home!:

  1. Put a bottle of unopened, store-bought mineral water in the freezer.
  2. Leave it in there for a week, or however long you think it will take for there to be no chance in hell that the water won't be frozen.
  3. Check the water. It won't be frozen solid!

What happened?!:
The mineral water was pure enough (hopefully) that there was too little or no particulate matter for the ice crystals to start building themselves on. Try taking the water out of the freezer and open the bottle. The water should immediately start to turn into firm, slushy ice. By opening the bottle, you have introduced dust from the air (particulate matter) into the supercooled water. Now the water (which is already below freezing point) has something to start forming itself on. Pretty cool, huh?!

If the experiment does not work, try it with another bottle of water. If it still doesn't work, consider trying another brand of bottled water. Maybe the brand you're drinking now isn't as clean as you think.