Glass is an amorphous solid, not a slow-moving liquid.

A popular myth holds that old windowpanes are thicker at the bottom because glass flows like an extremely slow liquid over the centuries. It isn’t true. Glass is a rigid amorphous solid: its atoms are arranged without the long-range order of a crystal, but they’re locked in place, not sliding past one another the way molecules in a true liquid do.

The uneven windows that fueled the myth are a manufacturing artifact instead. Before the float-glass process became standard in the 20th century, panes were often made by spinning or rolling molten glass into sheets, which left them naturally thicker along one edge. Old-world glaziers commonly installed that thicker edge at the bottom for stability, leaving buildings full of centuries-old windows that only look like they’ve sagged over time.