"Cecilia" (Simon and Garfunkel)

"Cecilia" and I go way back.  It was my favorite track from Bridge Over Troubled Water (1970) as a kid.  Its all-too-brief 2:55 would fly by, and I'd want to hear it again.  And again.  My mom was always reluctant to replay it, though—no doubt because of the lyric Making love in the afternoon with Cecilia, up in my bedroom...  But I had no clue at the time what Paul and Artie were singing about.  I just liked the melody and that crazy percussion.  It made me happy.  Still does.
The song began as an impromptu jam.  The duo was having a party at a rented house in the Hollywood Hills.  At some point, they started patting out this syncopated rhythm on their thighs, and then others joined in—playing the seat cushion of a piano bench, strumming the slackened strings of an old guitar, etc.  They liked the groove, so they put it to tape, using a home reel-to-reel recorder with a slap-back reverb feature (which is what gives the track its distinctive, ping-pongy ricochet sound).
Ultimately, they brought the tape to producer Roy Halee and asked him to make a loop out of a minute-long section that they thought had a particularly good feel.  By splicing sections of tape together, Halee turned one minute of tape into a literal three-minute loop.  Simon quickly came up with the melody and lyric about a guy whose girlfriend cheats on him but then comes back to him in the end.
"Cecilia."
The older I get, the less I think the song is about a flaky girlfriend, however.  At least not in the literal sense.  It's about music and songwriting.  
Cecilia is the patron saint of music and musicians.  Putting it in ancient Greek terms, she's a muse.  And as anyone who has tried their hand at songwriting knows, sometimes the muse is kind and makes you feel like the ideas are never going to stop flowing.  Other times, she's a fickle slut who comes and goes as she pleases.  In fact, sometimes she doesn't show up at all, leaving your self confidence in the gutter.  But when she comes back, and you're suddenly firing on all cylinders again, somehow, it's even better than the first time around.  All is forgiven and forgotten.  Jubilation!  
(That Paul Simon is a clever one.)