To begin with, let me quote from Wikipedia:
Earworm, a loan translation of the German Ohrwurm is a portion of a song or other music that repeats compulsively within one's mind, put colloquially as "music being stuck in one's head."
Urban Dictionary says: A song that sticks in your mind, and will not leave no matter how much you try. The best way to get rid of an earworm is to replace it with another. Be prepared to become a jukebox.
Assuming you read the above two paragraphs, which look boring at a glance but once you read it would make you say "Hey this happens to me all the time, you know what, just the other day I couldn't get a Justin Bieber song out of my head", I don't think I need to give a lengthy scientific explanation of what an earworm is. But, let me get back to boring once more. Wikipedia also says - Earworm may also refer to the Helicoverpa zea (corn earworm) or the musician DJ Earworm. Not to be confused with the creature depicted in Star Trek II: The Wrath of Khan.
I am least interested in worms that live in the ear, or a certain DJ, and I have never liked Star Trek. Star Wars I like, but sorry, not Star Trek. So since there is nothing else to do, let’s talk about earworms and songs that play inside one’s head.
Actually, I have no idea how to start. Conversations are not my thing. I could say “Studies have shown that there are certain songs which for some unknown reason tend to get themselves embedded in the frontal area of the brain which controls blah blah blah…” and I could look up and see my audience staring into space with a faraway look embedded on his face. If I say “Have you ever experienced that thing, you know, when you sing the same song over and over again in your head and sometimes you change songs without knowing it?” my listener would probably say a curt “Yes” and I’d be left with nothing to say except “Me too!!”
And sometimes you sing under your breath. This one is really weird. You are doing something, and at some point you suddenly realise that you have been singing for a while now, and you have no idea how you came to sing that particular song. It could be any song; it could be a song you listened to just an hour ago, a song you sang in church when you were a kid, or a song that your roommate plays all the time. You don’t even have to like the song. You have heard it so many times it has pitched a tent in your brain and set up camp there. Permanently. And once you look up from what you are doing and realise that you are singing, you immediately become self conscious. Hey why am I singing this song? Don’t I always wince and make a face whenever my roommate plays it? Then you stop singing, or change tracks. Kind of like the cartoon character who walks off a cliff but doesn’t know he’s walking on air until he looks down and sees nothing but air below him, and immediately crashes to the ground.