Supercooled water

There are a lot of videos like these on YouTube that show supercooled water. Here is what is happening. If you take very "clean" water (water with very few impurities, like bottled water) and have it inside a very "clean" container (like a plastic water bottle), and you lower its temperature slightly below freezing (like -6 or -7 degrees C), the water will sometimes supercool: It stays liquid even though it is below the freezing point. When you bump the water, a seed crystal forms and then the whole thing freezes in just a few seconds.

If it is -6 or -7 degrees C one night, you might try leaving several bottles of bottled water outside. Some of the bottles will freeze on their own, but normally a few will supercool and stay liquid.