Glass is not a slowly flowing liquid, and old window panes are not thicker at the bottom because they sagged.
The popular story says medieval glass has flowed downward over centuries. In fact, old panes were made by spinning or blowing glass, which produced uneven sheets, and glaziers often put the thick edge at the bottom.
Why is this true?
Glass is an amorphous solid: its atoms are disordered like a liquid but locked in place. Calculations show that glass at room temperature would take longer than the age of the universe to flow visibly.
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Science and nature supply facts that feel like they break the rules until you see the mechanism. Hot water freezing before cold, glass that flows, lightning hotter than the surface of the Sun. Each entry keeps the physics or chemistry short and readable, states plainly where the effect is still debated, and links to a source for anyone who wants the full derivation rather than the headline.