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Actually, it does! But only in some places where it's really, really cold, like the Arctic or Antarctica. Water is made of tiny molecules (MOL-uh-kyools) that move around a lot. As the temperature gets colder, the water molecules slow down. When the temperature falls to 32°F (F is short for Fahrenheit), these slow-moving molecules find each other and start sticking together to form ice crystals.
Ocean water is different from fresh water, which you find in lakes, ponds, and rivers. Ocean water has salt dissolved in it. Salt has small particles called ions that surround the water molecules and keep them from sticking together to form ice. Ice will only begin to form when the ocean water gets even colder—about 29°F.
As sea water gets colder, it gets denser. That means its molecules pack tightly together. Because it's more dense, this salty water begins to sink. That leaves less salty water on the top of the water. And this water begins to freeze. Sea ice has almost no salt in it. In fact, if you melt it, you can drink it!
In places that get super cold, like around the North and South Poles, there is a lot of sea ice at the surface—especially in winter. That doesn’t mean the whole ocean is frozen, though. Under the ice, the ocean water is still there. Otherwise, it would be very hard for seals and fish to swim, and for Arctic polar bears and Antarctic penguins to dive below the icy surface to hunt them.
More and more sea ice melts every year. To learn more about why this is important, watch this Discovery video on disappearing penguins.