It seems like a simple enough question - why are some materials transparent while others are opaque? But, as this video from the University of Nottingham's Sixty Symbols project explains, a lot of the most common explanations you get are fundamentally wrong. Watch a professional scientist recoil in horror at the terribleness of internet science, pretend to be a photon, and throw balls around his office, all in the name of explaining the energy gap that makes objects transparent or opaque.

[Via Sixty Symbols.]