Science & TechFact
No, old window glass hasn't flowed downward.
A popular myth says glass is a slow liquid, which is why old church windows are thicker at the bottom. In fact, glass is an amorphous solid. At room temperature, visible flow would take longer than the age of the universe. The uneven thickness comes from manufacturing: medieval crown glass was spun into a disc and never uniformly thick.
Useful: Glaziers usually placed the thick edge at the bottom, but sometimes at the top, which settles the myth.
Source: en.wikipedia.org