information guy learns physics - matter states

the labels of solid-fluid/gas are grounded in human perception, not physics. this is why these terms exist in every-day language as well.

these describe ranges on the scale of "fluidity over time". glass is called solid, but given enough time it will flow into another configuration (but since this time is so long for us mortals, we call it solid - perhaps a cleaner distinction would be calling things solid only when they would stay like that indefinite given no outside influences)

this is beacuse it has NOT reached the thermodynamic ground state under the set of current conditions of temperature and pressure (this is ignoring magnetic effects)

those usually look like lattices (crystals, where the atom configuration is symmetric/repeatable in any of the three dimensions), this is because the lattice is the most energy efficient form, when the kinetic energy of random thermodynamic movement (heat) is low. otherwise it is often better to have no structure at all (often gasses) or simply cluster (liquid). pressure shifts at which level it is.

but the change of these conditions is also what matters for what happens in practice. temperature and pressure changed too fast makes it trapped in a local minima (either one that is fake meaning enough time will bring it down at least a bit, or a real one where it will stay forever if conditions don't change - depending on whether the energy barrier around leaving that state is low enough that thermodynamic fluctuations and forces like gravity can overcome it). this happens with the prior mentioned glass that was cooled too fast, or diamonds.

interestingly enough, kinetic energy (KE) can also create the same situation where it traps a material in a state. sometimes permanent like "shock-amorphized minerals" , sometimes returning once the KE is removed like the experiment with the cornstarch-goo