Unlike other liquids, water expands as it freezes to become ice; so ice floats on water because ice has less density than water. This is very unusual - just about every other liquid gets more dense as it cools; water ice, however, is an important exception. Liquid water expands by about 9% as it becomes ice - it takes up more space. This is because the water molecules are actually farther apart instead of being closer together. The molecules in an ice crystal are arranged in a way that makes it less dense than liquid water.