I'm discussing hyperbole first in order to help clarify what I mean by the phrase "rarely articulated rules" in my title. Also, I beg forgiveness for any poor grammar. I'm trying to articulate a rather difficult concept in a limited amount of time so I may have let something slip. I hope my points gets across anyway.

Hyperbole is a figure of speech, constructed like a simile, but designed to draw attention to differences in order to exaggerate a likeness.  I think this a fair definition.  A good example of a hyperbole might be:  "foot as big as a barge."  In looking at that example, there seem to be some rarely articulated rules for constructing a hyperbole.  "Foot as big as a barge", for example relies heavily on the similarities of a foot and a barge.  Specifically, both a foot and a barge are longer than they are wide. It's the difference in proportion of the two objects that enables one to use this hyperbole effectively to describe the largeness of a foot by exaggeration. Therefore, the rarely articulated rule appears to be: The compared objects must have enough in common that what is different between them stands out. Let's try another example: "Foot as big as a mall". Since there really is no foot, whether on man or beast, that is anything similar to the shape of a mall (which would be hard to define in the first place), does this even qualify as a hyperbole? Or, is this just an ineffective hyperbole? Are there any other rarely articulated rules for constructing a hyperbole?

Metaphor This one really has me stumped. I understand the really easy examples like the ones where the metaphor is clearly a simile without the "is as" or "like". "Such a lovely rose that walks my way" seems a very clear metaphor to me. Culturally, so much can be assumed here that it seems clear a pretty lady is walking in the direction of someone admiring her beauty. Of course, I guess if this woman is rather ugly, sarcasm could be the real communication goal. But what about this one: "He's drowning in money". While I can see the similarities that are being drawn, there also seems to be a bit of an exaggeration here. I think it would be exceedingly rare to hear of anyone dying (drowning) as a result of being immersed in vast quantities of money. Are metaphors allowed to exaggerate? Or is this just an example of an analogy?

Are there any rarely articulated rules governing the proper construction of a metaphor? What is the purpose of a metaphor? Please don't tell me the purpose of a metaphor is "to draw similarities" between two things. I don't think that's the purpose of a metaphor. That seems more the function by which a metaphor accomplishes its' purpose. Could it be said, for example, that the purpose of a metaphor is "to bridge a gap" by drawing on similarities of two things that may seem quite different? What specific purposes do metaphors serve?

Why I'm Asking As a part of a debate argument, I was recently tossed a metaphor that drew on similarities between one thing that exists in the real world and another thing that exists solely in a very specific imagination, (it doesn't even exist in the "collective imagination" like, say, "pink elephants"). I'm wrestling with my instincts on this one. Somehow I have the notion that I've been handed an argument crafted with a linguistic tool designed for a far different purpose. But there are so many subtle nuances in the language I'm being handed that I'm convinced the answer to my problem lies somewhere in some rarely articulated rules for constructing metaphors - something similar to the "rarely articulated' rules for constructing hyperboles that I've mentioned above. ANYONE??? Do these rules exist? This seems the unique territory of a linguist, so I hope I've chosen the right forum.  Either an answer or a nudge in the direction of resources that could help would be great!  Thanks.