I just spent all week singing "Martha My Dear" by the Beatles under my breath. The week before, it was "I Like Fish" from Yo Gabba Gabba (my daughter's favorite show). But why?

As is all too often the case with the interesting parts of science, we don't know much about this phenomenon but we have a good name for it: earworm. People have been interested in earworms for a while now -- Mark Twain used one as a plot device in his 1876 story "A Literary Nightmare." They're the most common type of what's called "involuntary imagery," consisting of sounds, pictures, smells and even tastes that repeatedly come to mind.

One theory is that earworms are a form of mild musical hallucination (normally a rare experience), the distinction being that with an earworm you usually aren't on drugs or suffering from schizophrenia and are fully aware there's no actual music being played outside of your skull. Another theory is that earworms are a side effect of your brain trying to consolidate memories, akin to what happens in REM sleep. Neurologist Oliver Sacks tackles the subject in his book "Musicophilia," saying earworms might simply be a consequence of being surrounded by music in our lives whether we want to be or not.

A more promising line of investigation, in my opinion, is to focus on the earworminess of particular songs and their "cognitive itch." Certain pieces of music may have properties that excite an abnormal reaction in the brain -- in other words, your brain detects something extraordinary or unusual about the music that compels attention. Your brain tries to process the itch by repeating it, which only makes things worse. I find the music most likely to cause an earworm has one or more of three key qualities: repetition, simplicity and an unexpected rhythmic variation. The Pet Shop Boys' 1984 hit "West End Girls" is a great example.

A 2003 study by Billboard showed that nearly 98 percent of people experienced earworms, usually involving sung rather than instrumental tunes. This research also suggests that musicians and those inclined to worry are particularly susceptible to worm attacks. Despite all this, no one really knows for sure what causes earworms or how to get rid of them.

One final tidbit: A 2005 Rolling Stone survey found 7.5 percent of respondents were afflicted by their least favorite song as an earworm, and more than a third hated the song's lyrics more than anything else about it. The most loathed tune? Billy Ray Cyrus' "Achy Breaky Heart."

Now try to get that out of your head.