Density (usually abbreviated as *d* or *r*) is a mathematical concept used to describe the ratio between the mass of an object and its volume. The actual formula is the density times the volume is equal to an object’s mass, or *d* × *v = m.* In the standard (American) measurement system, density is measured in pounds per cubic foot. But in the sciences, the metric system is usually used and density is measured in grams per cubic centimeter (or grams per milliliter). For example, the density of water is 1 gram per cubic centimeter, lead is 11.3 grams per cubic centimeter, and gold is 19.32 grams per cubic centimeter. (Note: In the majority of cases, the higher the density, the “heavier” it feels to us on Earth.)