It's easy to take for granted the fact that glass is transparent. But stop and think about it for a second: how can something so bulky and solid be so easy to see through?
In fact, it might be better to ask not why glass is transparent, but why other materials are not transparent—which is exactly what Mark Miodownik, the great material scientist and communicator, does in this video.
As he explains, it's largely to do with the electron structures in the atoms that make up a material. If a photon of light is unable to provide the right amount of energy to an electron in a material to make it change state, then the photon simply passes through, unobstructed. And that's exactly the case for the photons passing through glass, hence its transparency: the light just pours through. [TED Ed]