The density of an object is equal to the mass of that object divided by its volume. Substances (like lead, water, iron, granite) have a certain density under normal pressures. In such cases the density of a substance can also be used to determine how much mass will be present given a certain volume of the substance. For example, water has a density of 1 gram per cubic centimeter (gm/cm3) so a cube of water 10 centimeters on a side weighs 1000 gm (1 kilogram). Some substances (like gases) are compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible (and hot!) gases and is much denser at its center than near its surface.