Glass has properties of a liquid. But do glass windows really flow over centuries becoming thicker at the bottom, as is commonly reported? Simple calculations show that the timet taken for a windowpane of heightL0 to increase in thickness byq% due to gravityg is given by
where the glass has viscosityμ and densityρ. For the small windowpanes common in medieval times this amounts to some millions of years! Thus, window glass behaves as a solid.