Icecool is right, songs are almost like living, changing beings. My friends and I get together weekly to jam. We started with nothing and came up with ten or so skeletons of songs (just a riff or two, or maybe a chord progression). Every time we play the songs change a bit, depending on how I'm feeling that night, or who's playing drums or bass, solos are always improvised, etc. For example, one song that was merely a driving 3 powerchord rhythm has now turned into a softer, melodic fingerpicked song.