What you learned in school
Chemistry and physics textbooks taught that glass was actually a liquid that flows extremely slowly over time. Students learned that old cathedral windows were thicker at the bottom because the glass had "flowed" downward over centuries. This seemed to explain the uneven thickness of antique glass panes and was presented as evidence of glass being liquid.
What we know now
Chemistry and physics classes often taught that glass is actually a liquid that flows very slowly over time. Students learned that old windows are thicker at the bottom because the glass has flowed downward over centuries. This explained why antique glass panes seem uneven. Glass is actually an amorphous solid - it has a rigid structure but lacks the crystalline organization of typical solids. The thickness variations in old windows result from manufacturing processes, not flow. Glass doesn't flow at room temperature; it would take longer than the age of the universe to see any measurable flow.