Someone once told me that glass, though it seems solid, is actually a very slow flowing liquid. If you examine the stained glass panels in medieval cathedrals, you'll find that they're slightly thicker on the bottom, supposedly showing how, over the centuries, the glass imperceptibly dripped from top to bottom.
However, while it turns out that the nature of glass is an improbably controversial (and, at least to me, all but impenetrable) topic, if it is a liquid, you certainly can't prove it by looking at old windows.
In fact, using the technologies of the time, medieval glass makers were simply unable to produce sheets of glass of a uniform thickness. And so they installed the glass thicker side downward because it balanced better that way.
And the moral of which is that, while, as Yogi Berra said, you can see a lot just by looking, sometimes what's most misleading is what's right in front of your eyes. It's like the marksman who amazed visitors by showing them a series of bullet holes in the side of his barn, each shot exactly in center of a bull's eye. When asked how he did it, he said, It's easy. You just fire a shot at the barn. And then you paint a target around it.
Have you ever been absolutely sure about something, but, you later learned, were totally, completely, entirely wrong?