The density of water increases as the temperature decreases, but only until approximately 4 degrees Celsius, where density starts to decrease at any point lower than that. When water freezes, turning into ice, it has a density lower than liquid water, which allows it to float. Ice cubes float on a glass of water like an iceberg floats on the ocean.
Density is an important physical characteristic of substances that measures how much matter is contained within a specific amount of volume. Density can be measured and compared for liquids, solids and gases. If a substance has a lower density than another it will tend to rise or float above the other.