When an ordinary piece of steel is put under pressure, the steel compresses; that is, its volume slightly decreases. Glass, however, is a fluid
rather than compressing, it flows when put under pressure; its volume remains unchanged. Any portion of a sheet of glass that is under sustained pressure will very slowly flow to areas under less pressure. Ttherefore, if a single, extremely heavy object is placed in the middle of a horizontal sheet of glass of uniform thickness and if the glass is able to support the weight without cracking, then the sheet of glass will eventually .
compress, although not as much as a piece of steel would