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Ice generally forms on microscopic scratches and bumps called nucleation sites. Nucleation sites, or 'bumps', occur within most containers with imperfections - even dissolved impurities such as calcium or even air bubbles act as nucleation sites for ice to form. If you remove the nucleation sites from the water, the water cannot form ice crystals easily.
Normally, pure water (without impurities) will form ice crystals at 0 degrees Celsius or lower.
However if pure water is cooled down very slowly, in a very smooth container with no dust inside, you can make water stay a liquid at temperatures below zero. Supercooled solutions are very unstable. Introducing air bubbles or a seeding crystal into the solution causes the liquid water to rapidly freeze.
Be aware that supercooling is different from freezing point depression. Freezing point depression occurs when you dissolve an ionic solid such as salt in water. A salt water solution will have a lower freezing point than pure water - do the experiment and find out for yourself!
The longer it takes to form a crystal, the larger the crystal will be.
Check out the link on crystals formed by volcanos! Indianapolis Childrens Museum