Pressure and Density
Because all objects are made out of molecules, it is possible to determine how tightly packed those molecules are. This is known as density. The more tightly packed the molecules of an object, liquid or gas are, the more dense we say they are.
The density of a solid object will remain the same no matter where we place the object. The density of a liquid will change only slightly. However, the density of a gas changes drastically.
A gas will expand to fill whatever space it is provided. Thus, if we take a certain amount of gas out of one container and place it into another container that is twice as large the gas will expand, filling the larger container. We still have the same number of gas molecules, but now, they are filling a much larger area. Thus, the gas is half as dense as it was, or in other words, there is twice as much space between the molecules as there was in the smaller container.