Voxels are a good help here. They are volumetric by nature, and we can always know what kind of material is there. Each material then can have its own specific weight. So wood is lighter than stone, and stone may be lighter than raw iron. These properties have a great deal of influence one how things interact with each other, also how they rotate, how their center of mass is computed.
Air is no different. It is meant to weight something. So what happens if you have a material lighter than air? Check out the following video:
We will be looking at the same principle for water in a next iteration.