Glass makes sounds! In addition to optical qualities, glass also has acoustic properties. Hollow glass produces a certain sound, when it is struck and made to vibrate. The custom of striking a glass with a spoon or something like it to make oneself heard in a group of loudly talking people is proof of it. Glass windows make a noise when you knock on them with your knuckles to attract attention. Glass was used in precious musical instruments as material for the keyboard or corpus. Every child tries out the effect of the glass harp moving wet fingers around the rims of glasses of different sizes/filling quantities.