I’m going to (hopefully) permanently end the popular misconception that glass is a liquid with this post. Glass is not a really slow liquid – it is in fact a solid, based on its flow properties (yay rheology!). But glass is a solid without structure, or in fancy terms, an amorphous solid. Many solids you see are crystalline, not just the pretty stones pop cultures tends to reserve the name “crystal” for. A crystalline material is one where their atoms or molecules are arranged in a repeating 3D pattern. The metals in your car, the silicon in your computer, and the calcium phosphate mineral in your bones are also all crystals because we can see their atoms follow some crystal structure. While the atoms/molecules can differ, mathematicians have found out that there are only about 200 distinct ways to make a repeating pattern in 3D with no gaps or overlaps (and only 17 for 2D). This might seem low, but the point is that while tiny details may change, there’s only so many ways to combine the symmetries you can find, like reflections or rotations, and still fill up all of space (or your wall).
The image above is a high-resolution transmission electron micrograph literally showing you the atoms in silica (silicon oxide) – the material that makes up regular glass. On the left, it’s a crystal and the dots on the bottom show you in red and green where the different atoms are in a hexagon arrangement. Around 3/4 of the way to the right you see that the atoms are no longer always in hexagons and the shapes start to change. This side is amorphous.