Ice floats in water because, unlike most substances, the solid phase of water is less dense than its liquid phase. When water freezes, it expands slightly, and the resulting change in density is enough to allow ice cubes to float in a glass of water.
Ice's expansion at its freezing point is due to the polar structure of water molecules. In the liquid phase, water molecules are always in motion and can be packed closely together. As the temperature drops, however, the molecules begin to form hexagonal crystals. The hydrogen bonds created in these crystals arrange the molecules in such a way that more space exists between them than in the liquid phase, resulting in an expansion of around 8.3 percent in volume. Since the water the ice displaces now weighs less than the ice itself, ice is buoyant and floats.
This unique property is important to the development of life on earth. Because ice floats, rivers and streams form a layer of ice during winter. If water behaved like most other solids and became denser when cooled, this ice would fall to the bottom of each body of water, freezing it solid and killing any living things under the surface.