Songs that hinge on central metaphors typically fasten themselves to a familiar structure: the first verse introduces the situation, the chorus introduces or solidifies the metaphor, and further verses elaborate on the metaphor while further choruses act as a grounding device to keep everything safely within the bounds of the metaphor. Brick by Ben Folds Five is a terrific example of this.
Which brings us to Judy Is Your Viet Nam by They Might Be Giants, and the interesting way it approaches its central metaphor.
The Discerning Lyricist 