> The Myth of Flowing Glass
This one gets repeated at literally every cathedral tour with old stained glass, and correcting it never stops being satisfying.
The claim
Old window panes, medieval church glass especially, are often noticeably thicker at the bottom than the top. The popular explanation: glass is technically a liquid, not a solid, and over centuries it slowly "flows" downward under its own weight, like unbelievably slow syrup. It's a genuinely elegant explanation. It's also not true.
Why it can't be true
Glass does have an unusual molecular structure. It's an amorphous solid, not a crystalline one, which is where the "it's secretly a liquid" idea gets its foothold. But the actual physics doesn't support visible flow on any human timescale. Viscosity research on real medieval glass (published by Gulbiten et al. in the Journal of the American Ceramic Society, 2018, among other studies) found medieval glass is actually about sixteen orders of magnitude less viscous at room temperature than earlier estimates assumed. Even with that correction, the maximum plausible flow still comes out to roughly one nanometer over a billion years. The universe itself is about 13.8 billion years old. Do the math and a millennium-old church window hasn't had remotely enough time, even in principle, to sag by an amount anyone could ever see.
What's actually going on
The real explanation is almost embarrassingly simple. Medieval glassmakers using crown-glass or cylinder-glass techniques couldn't reliably produce a perfectly flat, uniformly thick pane, so the process left panes naturally thicker on one edge. When glaziers installed those panes, instinct did the rest: the heavier, thicker edge went at the bottom of the frame, because that's the more structurally sound way to seat an unevenly weighted piece of glass. The glass didn't move there over time. Someone put it there on day one, for a completely mundane, completely sensible reason.
How the myth got so far
This isn't some fringe belief. It's been repeated in real high school science textbooks and even, at points, the Encyclopedia Britannica. Part of how it spread that far: an influential early paper on glass viscosity by physicist Gustav Tammann appears to have gotten misread or mistranslated somewhere along the way, lending the "secretly still flowing" idea a false patina of citation. Once a wrong explanation sounds technical enough, it tends to outrun the boring correct one, which, ironically, is the actual physics.
