Water expands by about 9% when it freezes into ice.

In liquid water, molecules move around and stay close together. Water is heaviest and most dense at 4 degrees Celsius (39.2 degrees Fahrenheit). As it gets colder, the molecules slow down. They begin to form steady connections called hydrogen bonds.These bonds push the water molecules into a hexagonal shape like a honeycomb. This shape has a lot of empty space in the middle. Because of this space, the ice takes up more room than the liquid water did. This expansion creates a lot of pressure. It can reach over 30,000 pounds per square inch.This pressure is strong enough to burst metal pipes or crack giant rocks. Because ice takes up more space but has the same weight, it is less dense than water. This is why ice floats. If ice sank, lakes would freeze from the bottom up and kill fish. The floating ice acts like a blanket to keep the water below warm enough for life to survive.