Old windows are thicker at the bottom because glass slowly flows
Look closely at medieval window panes and many are thicker at the bottom. The popular explanation is that glass is really a liquid, sagging over the centuries. Is that what happened?
Myth or fact? Reveal
Myth. At room temperature glass is far too stiff to flow: physicist Edgar Zanotto calculated that a visible sag would take longer than the age of the universe. Medieval panes were simply made uneven, and many were set in their frames thick edge down.
— Edgar D. Zanotto, Do cathedral glasses flow? — American Journal of Physics, 1998