This perception is based on years of misguided elementary school teachers taking students on tours of colonial buildings and being told that because we see the glass is thicker at the bottom on old houses it is because the glass has "flowed" even in its solid state.
In truth it's because to make flat glass at that time glass was spun on a wheel until it flattened out like a centrifuge would cause it to, and then rolled flatter with a weighted roller. The side effect of this process was squares of glass being cut out of a large circle whose outside edge was thicker form the spinning process.
Two different post-dco students have used different techniques to demonstrate the rate of actual "flow" of material in glass at a solid state. To produce a difference noticeable with the naked eye in glass at room temperature it would take longer than the current age of the universe.
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