Water expands by a factor of 112, or roughly 9 percent, when it freezes. Water, unlike most other liquids, forms stronger hydrogen bonds as temperatures drop. This leads to a crystal lattice structure with voids left between molecules. As the molecules arrange themselves in this manner, the substance expands and becomes less dense.
Water molecules normally slide passed one another fluidly and easily, but when the temperature drops to below freezing, the two hydrogen atoms align with their neighbors. This alignment eventually becomes a rigid, crystal structure, leaving openings between the molecules. These open areas are where the overall volume of the substance has increased, or expanded, by approximately 9 percent, which is why ice floats on water and does not sink.
If water did not expand when frozen, Earth would be a different place. If water contracted when frozen, it would be more dense than the surrounding water and sink. This would leave ice submerged and out of reach of sunlight, while freezing fish and animals living at or near the bottom. The submerged ice would also slowly cool the planet, leading to a cold and inhospitable environment for life as it is known today, particularly so for human life.