1. What is a glass?

1.1 A little physics

How can a gas, liquid or solid be represented at the level of its constituent atoms or molecules? A gas is a collection of atoms or molecules that move freely within the enclosure that contains them. When a gas is cooled to condensation temperature (e.g. 100°C for water vapor), it turns into a liquid. In the liquid, the atoms or molecules are in contact with each other, but without chemical bonds between them, allowing the liquid to deform, rather like the contents of a bag of marbles. If, finally, the liquid is cooled, it congeals into a "crystallized solid": this time, not only are the atoms or molecules linked to each other by chemical bonds, so they can no longer slide over each other like the marbles in the bag, but they are arranged in a given order. This...