The big idea
Glass is strange. It is a hard solid, yet light passes through it almost as if it were not there. It is made mostly of silica, the main chemical in sand, and it can be shaped into windows, bottles, lenses and the screen of a phone.
How it works
In most solids, such as salt or metals, the particles are packed in a neat, repeating pattern called a crystal. Glass is different. It is an amorphous solid: "amorphous" means "without shape", and its particles are jumbled, a bit like a crowd frozen in place. Glass forms when the melted mix cools too quickly for its particles to line up.
Why can you see through it? Sand looks cloudy because each grain has edges, and light bounces off every edge and goes off in all directions. Solid glass has no grains and no edges inside it, so light travels straight through. The particles in glass also do not soak up the colours of visible light, so very little of it is lost.
Glass is a true solid, not a very slow liquid. Some old church windows are thicker at the bottom, but that is because the old ways of making glass gave uneven panes, and builders often set the thick edge at the bottom.
How it's made
The raw materials are sand, soda ash (which lowers the melting temperature) and limestone (which stops the glass dissolving in water). Crushed recycled glass, called cullet, is added too. A furnace melts the mix at about 1,500 °C.
Most window glass is made by the float glass process: