Why Light Ruins Your Beer

Via Wired:

Ultraviolet rays break the bond connecting the central isohumulone ring to a side chain of oxygen and carbon molecules. This newly released molecule (called a free radical) has a spare electron, which is desperate to bond with any other available molecule. Floating in the beer are many potential partners, most of which are safe from a taste perspective. However, when a free radical mingles with a certain sulfur alcohol molecule (called sulfmethyl) present in small amounts in beer, the two will combine into something that smells a lot like an angry skunk.