Eventually folks found that substances could be heated to such extremely hot temperatures that they no longer behaved as gases. These they named plasmas. Plasmas are found most commonly in places like stars and magnetic bottles. At the other end of things, some substances can, if chilled to a low enough temperature, display a very altered behaviour: electrical resistance breaks down, electrical conductivity is complete. As solid metals, even the best electrical conductor, silver, has some loss of energy as heat. Not in these superconductors; they were deemed to be different enough from their solid counterparts to merit a separate classification: superconductors.

In some more recent chemistry circles there has been a move to redefine the old three states of matter, not by behaviour but by structure. Solids were replaced by crystals. Where solids hold their shape, crystals have a definite lattice structure that keeps the atoms or ions in position. Liquids weren’t much changed in concept, but were broadened to include the things solids lost in becoming crystals. The best known of these is glass, which does not have a crystalline structure. I am told, though I have never tried it myself, that if you lay a plate of standard window glass flat and stand a pencil, tip down on it, you can come back several decades later and the pencil will be penetrating through the glass. More reasonable, perhaps you’ve noticed how in old windows in an old house the glass is warped and distorts the view. It is undisputed that the glass at the bottom of these windows in thicker that at the top. Whether this is because of the flow over the decades or because that’s the way the glass was made originally way back then, is a matter of some contention. In any case, though, over the very long term glass flows - and thus should be considered not a solid, but a very, very viscous liquid.

The point, and no I haven’t forgotten it, is that most hard things are crystalline - they have a basic lattice structure that holds the atomic/ionic particles in place. This is why they are hard. Materials that are not hard have weaker, or no lattice structure. A little further in these studies we’ll develop the point that the degree of hardness is related to how firmly they are held in position (and, more fundamentally, why).