Okay, I've got one, too:
A drummer plans to study music and so he's at the university to do a test. The
instructor starts playing C and E, and asks what this is. The drummer just
replies, "play it again, please." So the instructor plays again C and E. The
second time the drummer, having no idea, replies "I think I've got it, but
play it again please, just to be sure." Again, C and E. Suddenly, the drummer
realizes: "It's a piano!"