Glass is a solid — it doesn't slowly flow

You may have heard that glass is really a slow-moving liquid, and that medieval cathedral windows are thicker at the bottom because the glass has gradually flowed down. It’s a tidy story, but it’s wrong.

Glass is an amorphous solid: its atoms are locked rigidly in place, just without the orderly, repeating pattern of a crystal. That disordered arrangement is what makes it both transparent and genuinely solid.

The atoms do move, but unimaginably slowly. By one estimate it would take longer than the age of the universe for room-temperature window glass to visibly sag.

The window isn’t melting — it never was flat.

The real reason for uneven old panes is manufacturing. Early glass was spun or blown into sheets of varying thickness, and installers simply tended to set the heavier edge at the bottom. Far older Roman glass survives with no sign of flow at all.