When water freezes, the molecules slow and settle into place, lining up in regular formations you see as crystals. For pure water, the temperature must drop to 32 degrees Fahrenheit (zero degrees Celsius) for this to happen. For any substance, the temperature at which freezing occurs depends on forces that make its molecules stick together. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.