Probing Question: What makes a song catchy?

A catchy tune isn't always a good thing. If it contains an infectious chorus, even a song you dislike might refuse to leave your head. ("Achy Breaky Heart," anyone?) The choice of descriptors is not lost on Keith Duffy, a professor of rhetoric and composition at Penn State Schuylkill and a musician on the side. "Oddly enough," Duffy said, "this same rhetoric also describes communicable diseases."