Volume is the amount of space an object occupies.

This seems a simple concept, and it is for basic, solid objects. For a solid cube, for example, multiply length by width by height to obtain the volume. But what is the volume of a gas, which can easily expand and contract? Except for idealized mathematical models, the volume of an object depends on the composition of that object, and the environment around it. Scientists have spent significant effort to formulate how such quantities depend on their environmental quantities like pressure and temperature. The volume of a solid, for example, typically expands slightly as the temperature increases, but in general they are mostly resistant to changes in their shape or volume. A liquid like water will take the shape of whatever container it is poured into, but its volume stays the same. The gas in a balloon will take...