Memory strength is also a function of how much we care about the experience. ...If I'm playing an instrument I like, and whose sound pleases me in and of itself... caring leads to attention, and together they lead to measurable neurochemical changes. Dopamine, the neurotransmitter associated with emotional regulation, alertness, and mood, is released, and the dopaminergic system aids in encoding the memory trace.

Paul McCartney may be the closest thing our generation has produced to Franz Schubert -- a master of melody, writing tunes anyone can sing, songs that seem to have been there all along. Most people don't realize that "Ave Maria" and "Serenade" were written by Schubert (or that his "Moment Musical in F" so resembles "Martha My Dear"). McCartney writes with similar universality. His "Yesterday" has been recorded by more musicians than any other song in history. Its stepwise melody is deceptively complex, drawing from outside the diatonic scale so smoothly that anyone can sing it, yet few theorists can agree on exactly what it is that McCartney has done.

One hundred years from now Beatles songs may be so well known that every child will learn them as nursery rhymes, and most people will have forgotten who wrote them. They will have become sufficiently entrenched in popular culture that it will seem as if they've always existed, like Oh Susannah, This Land Is Your Land, and Frère Jacques.