Glass appears to be a solid, but in theory it sometimes behaves more like an extremely slow liquid. Physicists in Utrecht now show that glass-like structures can also exist in equilibrium, which is something many theories say should be impossible.

The bottom parts of medieval window panes, such as those in old cathedrals, are often thicker than the top. Has the material slowly flowed downward over the centuries, and does this mean that glass actually flows? This is a persistent myth, and the explanation lies in the way glass was produced in the Middle Ages. Because window panes were made by hand, their structure was often irregular and contained thinner and thicker parts. The panes were usually installed in the frame with the thicker side at the bottom, which made them more stable.

Still, the story touches on a real physics question. What glass actually is, a solid or a very slow liquid, turns out to be more difficult to answer than it seems.