Glass is a hard, transparent material made by heating sand and other ingredients to extremely high temperatures until they melt together, then cooling the mixture rapidly. When you look through a window, drink from a drinking glass, or wear eyeglasses, you're using this remarkable invention that lets light pass through while keeping wind, rain, and cold air out.

People discovered how to make glass thousands of years ago, probably by accident when sand near a very hot fire melted and cooled into a clear, hard substance. Ancient Egyptians made glass beads and bottles, but glass remained rare and expensive for centuries. The ability to make large, flat panes of window glass changed architecture forever, allowing buildings to be filled with natural light while staying warm and dry inside.

Glass breaks easily when struck hard, shattering into sharp pieces, which is why we say something fragile is “as delicate as glass.” But glass is also surprisingly strong in other ways. The thick glass in aquarium walls holds back thousands of gallons of water, and specially treated glass in car windshields can withstand tremendous force.

Scientists value glass for laboratories because it doesn't react with most chemicals, and you can see what's happening inside a glass container. The word glass can also refer to a drinking vessel made of glass, like when you ask for “a glass of water.”