This morning I found myself staring at the old window in the library, running my finger along the uneven glass. The bottom pane was noticeably thicker than the top, and I caught myself almost repeating the myth I'd heard a dozen times: that glass is a slow-moving liquid, flowing downward over centuries. A colleague walked by and said, "See? That's why medieval windows are always thicker at the bottom." I wanted to correct her, but I hesitated. The myth is so persistent, so intuitively appealing.
Here's what's actually happening: glass is an amorphous solid, not a supercooled liquid. When it cools from its molten state, its molecules freeze into a disordered arrangement rather than forming a crystal lattice. They don't move. Old windows are thicker at the bottom because of manufacturing inconsistencies in crown glass production—glassmakers simply installed the heavier side down for stability. That's it. No flow. No creeping molecules.
I like to compare it to a snapshot versus a video. A crystalline solid like ice has molecules arranged in neat, repeating patterns—a carefully composed photograph. Glass is more like a frame grabbed from a chaotic video, frozen mid-motion. But once frozen, it stays frozen. The confusion comes from the word "glass" itself, which scientists sometimes use to describe the state of a material, not just the window kind.