Twist and Shout by The Beatles I had learned to play the guitar as a child, between perhaps 6th and 8th grade. I could read music to some degree, playing terrible little songs in the Mel Bay book series. I had a patient and talented guitar instructor, but the truth was that I had no real understanding of the rudiments I could write down and perhaps recite back to him. I put away my guitar in disgust when a friend in a band developed the ability to play songs by ear -- a vital talent and one that required the true understanding I lacked.
One day in around 11th grade, I pulled the old Stratocaster out and looked at my old handwritten notes I'd taken during one of my last lessons 3 years ago. I saw in my own hand, something I'd written about there being three chords in a given major key, and it suddenly made more sense than when I'd written it. I remember grasping an A7 chord on the guitar and strumming through and immediately hearing in my mind: "well, shake it up BABY now...". I looked at the notes and tried other chords, and before too long at all, I could play one of my favorite songs.
I never became a great musician, but I have been deeply enriched by the moment I became conversant in the art form. I'll never forget it.