WRONG: Glass is a liquid

Observing church windows, many people have noticed that ancient panes are thicker at the bottom than at the top, leading them to believe that glass has “flowed” downward with imperceptible slowness.

Balls.

Medieval glaziers lacked the technology to make perfectly flat panes. Their methods, which involved spinning flattened, blown spheres, produced sheets of glass that were slightly thicker at one end. The heavier end was placed at the bottom of the frame simply because it’s more stable that way. The best medieval English glass was made from Lyme Regis sand and burned kelp. As was the typical medieval English peasant's lunch.

When glass is heated beyond its melting point, it becomes a liquid (because, well, that’s what melting means). Its molecules slide all over the place in disarray. As it cools, its viscosity increases, making it harder for the molecules to line up in an orderly, crystalline form. Compare this with water, the viscosity of which does not increase as much when its temperature lowers – at the freezing point, it crystallises into ice. (Water does do some weird things in extreme conditions, though. Look at this and this.)

The resulting hodge-podge of glass molecules is an “amorphous solid” – the molecules are bound tightly together, but not in a formal structure. Having said that, amorphous solids can flow – but for glass to slip in the way perceived in old windows would take millions of years and a great deal more heat than the average cathedral boiler provides.