The Beatles - Yesterday

The rerelease of the Beatles catalog along with a signature edition of Rock Band has stirred up more Beatlemania than I've seen since the Beatles Anthology documentary series in 1995 (I'm too young to have witnessed the real thing in the 60's). Whether you like them or not, there is no denying the impact that they've had on several aspects of the musical world, including fashion, marketing guitars and, of course songwriting.
Yesterday isn't my favorite Beatles song and I probably wouldn't even rank it in my top ten, but i'm not naive to it's influence on pop music, let alone the emotional command it has on listeners. It's also the most covered song of all time according to BMI.
I'm also fascinated by the story of how Paul McCartney came up with, then evolved the song.