Comment Glass is not a slow-flowing liquid (Score 2, Interesting) 293
Window glass used to be made by blowing giant glass discs and cutting rectangles or diamonds out of them to piece together to make leaded windows. The method of blowing the glass discs resulted in glass that was often thicker on one side than the other. The person building the window would naturally orient the thicker side of each piece to the bottom of the window, to work with gravity to make the window stronger and longer-lasting.
If the glass were really a flowing liquid, then the edges of the pieces would be rounded and deformed, but they are not. They are as sharp and straight as the day they were cut.