This is a common fact. If you have heard a specific song, say, Fireworks, it is usual and completely normal that you can still hear it playing somewhere in the back of your conscience. If that song didn't strike you as a nice one or something you would like to hear again and again, you would forget it in a day maximum.
But if the song hit you like it was sent straight from heaven especially for you, the song would keep on repeating and specifically the few verses you love. If you repeat the song too, the song would be impossible to get rid of. I know I am dragging on the paragraph but of-course, I cant turn this delightful piece of information into a formal class lecture. So I must take these precautions. So... on to the point!!!

For most of the 200,000 years of modern human evolution, facts, history, processes and other information were transmitted and remembered through spoken and sung words. This has led some scientists to opine that the human brain has become hard-wired to encode spoken and sung information, and recall it upon demand. Consider that, even today, many find it easier to memorize something to which a rhyme or rhythm has been added.

Whether we were already heavily predisposed to this sort of thing or it’s been a relatively recent development in human history, the hard-wired brain theory builds on the fact that music is “multi-sensory” and asserts that while the notes and lyrics are being memorized, the feelings and ideas the music also triggers are stored along with it. Then later, when that feeling or idea is remembered, it sometimes also brings a catchy portion of the song back up with it, in what is called “involuntary memory.”

From here, our brains seem hard-wired to finish what they start with the portions of the songs. In fact, when researchers at Dartmouth played small snippets of familiar songs to research subjects, they found that the auditory cortex of the brain continued the song in the subjects’ heads even after the music stopped. It’s perhaps not surprising from this that the more you listen to a song, the more likely it is that at some point it will stick in your brain. Further, people with Obsessive Compulsive Disorder have been shown to be more prone to getting songs stuck in their heads. 

So now. See? That's how we can do the impossible!!!!