Quanta Magazine:

It’s the mystery posed by every window and mirror, every piece of plastic and hard candy, and even the cytoplasm that fills every cell. All of these materials are technically glass, for glass is anything that’s solid and rigid but made of disordered molecules like those in a liquid. Glass is a liquid in suspended animation, a liquid whose molecules curiously cannot flow. Ideal glass, if it exists, would tell us why.

When you cool a liquid, it will either crystallize or harden into glass. Which of the two happens depends on the substance and on the subtleties of the process that glassblowers have learned through trial and error over thousands of years.

Exactly why the cooling liquid hardens remains unknown. If the molecules in glass were simply too cold to flow, it should still be possible to squish them into new arrangements. But glass doesn’t squish; its jumbled molecules are truly rigid, despite looking the same as molecules in a liquid. “Liquid and glass have the same structure, but behave differently,” said Camille Scalliet, a glass theorist at the University of Cambridge. “Understanding that is the main question.”

I had no idea there was any such thing as “ideal glass” and the line, “Glass is a liquid in suspended animation” kind of blew my mind. It’s one of those things you know but didn’t know.