Window glass is not a very viscous liquid, it is an amorphous solid.
Why it is true
The myth that glass flows over the centuries comes from old windows that are thicker at the bottom. This happened because the manufacturing methods of the time created glass sheets with irregular thicknesses.
Source: Corning Museum of Glass
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.