Not all metaphors are the same. What are some ways of distinguishing them?
The most common is the Dead vs. Poetic Metaphor.
I extrapolated from that to Cache Lookup vs. Calculation. (Using software as a metaphor for how metaphors work.)
However, as someone suspicious of Binary Oppositions and with a deep belief that Your Body is a Gross Kludge, I've come to a Kludge Theory of Metaphor.
Couple that with the Expensive Brain, and my theory (it is mine 
) is that when the brain encounters a metaphor, it attempts a cache lookup. In physical terms, some minimal set of Networks light up.
If the cache lookup fails, the brain lights up more networks in an effort to extract meaning from context. (See Learning Balls to the Wall.)
It's in the nature of brain Networks that they're likely to stay lit up for a while after they're no longer needed. So if the brain starts to *use* the meaning of the metaphorical statement – for problem-solving, perhaps – those networks can be enlisted to do further work, which likely involves lighting up networks associated with them.
So *what happens* to a metaphor in the brain is (I claim):
1. heavily driven by the details of what the brain is trying to accomplish, and
2. not as heavily driven by what "natural kind " the metaphor belongs to.
Might it be useful to think of metaphors as being akin to affordances ? Metaphors as Affordances TBD