It specifically needs to be a song -- the words must scan, must fit the rhythm and meter of the music. They don't need to rhyme -- there's no shortage of interesting, good songs that have odd or non-existent rhyme schemes -- but it must be something singable, sung with appropriate music.
Oh, and as thrashed out long ago, different versions by different artists have varying effects, although they all tend to cluster around the same general "theme". Different versions by the same artist, as long as the orchestration/arrangement doesn't change dramatically, have identical effects. "Canon" example: As noted in chapter, um, 2, I think, Doug prefers to use the "bootleg concert version" of "On The Dark Side" instead of the album single because, among other reasons, it lasts longer; both have the same effect, though.
-- Bob
---------
Then the horns kicked in...
...and my shoes began to squeak.
Oh, and as thrashed out long ago, different versions by different artists have varying effects, although they all tend to cluster around the same general "theme". Different versions by the same artist, as long as the orchestration/arrangement doesn't change dramatically, have identical effects. "Canon" example: As noted in chapter, um, 2, I think, Doug prefers to use the "bootleg concert version" of "On The Dark Side" instead of the album single because, among other reasons, it lasts longer; both have the same effect, though.
-- Bob
---------
Then the horns kicked in...
...and my shoes began to squeak.