Most of us will have heard that because glass is a supercooled liquid,* or an extremely viscous liquid, or an amorphous solid, or anyway a material with some very odd properties, old panes of window glass will gradually, over time, flow downward and become thicker at the bottom. Have we also heard that this is in fact untrue?
There are several ways to prove it, ranging from hm&te scientific demonstrations, to the simple observation that whereas old glass windowpanes do tend to be thicker at the bottom and thinner at the top, no one has ever spotted an instance where the supposedly flowy glass has actually flooped out over the edge of the framing material.
In my opinion, this one’s simple. Old manufacturing methods produced glass of irregular thickness. If you were a glazier, almost every pane you installed would have a thicker end and a thinner end. The lower portion of the pane bears more weight, so you put the thick end at the bottom. Light normally comes from above, so you put the thinner and more transparent end at the top. The arrangement is no more evidence that glass flows, than tapered wooden shingles are evidence that wood flows.