Why can gases pass through each other while solids cannot? Solids cannot (generally speaking) pass through each other, while gases can. The first answer to this question points to the electromagnetic force and the Pauli Exclusion Principle as the reasons that solids cannot pass through each other. However, the discussion I've read on this topic while digging around on it generally seems to apply as much to gases as to solids: for example, if solids are not "mostly empty space" then is the same true of gases, which are less dense but not necessarily enormously so? Why are gases different? It's true gases are less dense, but the difference does not have to be large near transition points. What are the forces or other phenomena that make solids coherent, and why are those forces etc. critical for it being impossible to walk through walls? In particular, is it fair to say that there is an attractive force at play, and if so what is that force?
 A: Solids don't pass through each other because the molecules are rigidly connected to each other and the "empty space" is not really empty, it is full of molecular wavefunctions.  As described in the referenced answer to Why doesn't matter pass through other matter if atoms are 99.999% empty space?, solids cannot pass through each other without the electron wavefunctions overlapping and repelling each other because of electromagnetic forces and the Pauli Exclusion Principle. To pass through each other they would have to break the rigid chemical bonds between molecules.
In a typical gas, the molecules are not connected and most of the volume really is empty so molecules rarely touch and can easily pass each other without their wavefunctions overlapping except during collisions.
Perhaps the more interesting case is a dense liquid, where the distances between molecules is similar to that in a solid, but the molecules are not rigidly connected to each other.  The fluids can pass (e.g. diffuse) through each other, but with difficulty since the molecules have to push each out of the way. There may also be weak molecular bonds that keep different liquids together as they pass through each other, such as happens with oil and water.
Think about a sidewalk with of people walking in both directions.  If there are few people (a gas), people walking in opposite directions can pass by each other without slowing down. If the sidewalk is crowded (a liquid), people can still move by each other but slowly since they have to walk around each other. If people going in each direction link arms (a solid), nobody can pass each other.
