Density is a measure of how much mass a substance has in each volume. The denser a substance is, the more mass it has in each volume. The less dense a substance is, the less mass it has in each volume.

Water is one of the most common substances on Earth and its density varies depending on temperature. At room temperature, water has a density of 1 gram per milliliter (g/mL). This means that for every 1 mL (milliliter) of water, there is 1 g (gram) of mass. As the water gets colder, its density increases because the molecules slow down and pack together more tightly. For example, at 4 degrees Celsius water has a density of 1.000 g/mL while at 0 degrees Celsius its density becomes 1.039 g/mL. On the other hand, as the water gets hotter, its density decreases because molecules move faster and take up more space. For instance, at one hundred degrees Celsius water vaporizes into steam with exceptionally low density.