Virtually everyone experiences them, and rarely are they thought of fondly. They are earworms, the tunes that burrow into our consciousness and play on repeat. In a recent study involving hundreds of people, Ira Hyman Jr. of Western Washington University and colleagues looked at what made songs most likely to stick, exposing unsuspecting subjects to popular songs and then asking them to complete various tasks. Previous research showed that people can recall the first verse of a song they like, but after the chorus stumble over the lyrics. At this point the song becomes incomplete — a conflict without closure — and that is one way that it becomes an intrusive thought, Dr. Hyman said.