Glass is a strange material, with properties that resemble both solids and liquids. Scientists call glass an amorphous solid. Amorphous is a Greek word that literally means without shape. Glass, of course, can take on many shapes; from windowpanes to beer mugs. But at the microscopic scale, atoms in glass are randomly distributed and have no particular arrangement. Other solids, such as crystals, have orderly molecules that line up in precise orders.
Liquids are also amorphous, and many scientists consider glass as a type of very thick liquid. However, in addition to being randomly distributed, the molecules in a liquid move about easily. The molecules in glass, on the other hand, are strongly bound in their random positions and hardly move about at all.
Most glass in windows and household items is made primarily of silica, which is also the chief component of beach sand. At temperatures of a thousand degrees Fahrenheit or more, the molecules in silica glass begin moving enough that the glass becomes truly liquid. At lower temperatures, and certainly at room temperature, glass molecules are rigidly frozen in place.
Some people claim that glass in very old window pains shows signs that it has flowed over the course of hundreds of years, which they believe proves that glass is a liquid. In fact, scientists have found that glass is so stable that it would take more time than the universe has existed for it to flow by a detectable amount. It's more likely that variation in the thickness of old windows is the result of older glass manufacturing processes.
