Interesting article from How Stuff Works. (Read full article)
Why do songs get inextricably stuck in our heads? Experts say the culprits are earworms (or "ohrwurms," as they're called in Germany). No, they're not parasites that crawl into your ear and lay musical eggs in your brain, but they are parasitic in the sense that they get lodged in your head and cause a sort of "cognitive itch" or "brain itch" -- a need for the brain to fill in the gaps in a song's rhythm.When we listen to a song, it triggers a part of the brain called the auditory cortex. Researchers at Dartmouth University found that when they played part of a familiar song to research subjects, the participants' auditory cortex automatically filled in the rest -- in other words, their brains kept "singing" long after the song had ended. The only way to "scratch" brain itch is to repeat the song over and over in your mind. Unfortunately, like with mosquito bites, the more you scratch the more you itch, and so on until you're stuck in an unending song cycle.