"What is it about the music you love that makes you want to hear it again? Why do we crave a "hook" that returns, again and again, within the same piece? And how does a song end up getting stuck in your head? Whether it's a motif repeated throughout a composition, a sample looped under an electronic dance beat, a passage replayed incessantly by a musician in a practice room-or an "earworm" burrowing through your mind like a broken record-repetition...