Density is a measure of how much mass is contained in a given volume. It is calculated by dividing the mass of an object by its volume, often expressed in units like grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). For example, a small rock may have a high density because it is heavy for its size, while a large balloon has a low density because it is light and filled with air. Different materials have different densities, which can affect how they behave in various situations. For instance, when you drop a wooden block and a metal block in water, the wooden block floats due to its lower density, while the metal block sinks because it has a higher density. Understanding density helps in fields like engineering, chemistry, and physics.