Technically, yes. Unlike a substance like water that changes from a liquid to a solid when frozen, glass develops no crystalline structure to maintain a rigid structure. For decades, science textbooks have supported this fact by pointing to the windows of medieval churches. Those window panes are often thicker at the bottom, and popular wisdom was that they had “flowed” downward at a very slow rate. In the late 1990s, however, some scientists decided to actually test that theory. They confirmed that, yes, glass would in fact exhibit some flow over time, but that it would take at least 10 million years for any visible change to occur in those window panes. It turns out that the sagging medieval glass, from mere hundreds of years ago, is only evidence of the period’s primitive glassmaking techniques.