How do songs get stuck in one's head?

I’m just curious, as far as brain regions and mechanisms go, why do songs get stuck in your head? What scientifically makes a song catchy? I mean, my girlfriend and I have had the new information number here in France stuck in our heads for days…“ceeeeeent dix-huit, deux cent dix-huit!”…why do things get stuck in your head and you can’t stop singing them over and over again?

you can get Sean Bennett’s thesis on this very subject here:

http://www.seanbennett.net/music/essays.html