Writerly Notions: metaphors & comprehension

The complex aspect about metaphors — or any kind of literary technique used to compare or create meaning — is that there’s a second level of comprehension needed.

For example, the book I’m reading on how to write short stories cites a cliche metaphor of “sharp as a tack” (102). For me to get the meaning — namely that whoever is “sharp as a tack” is, I presume, very smart (is that right?) — I have to connect the idea of the sharpness of a tack to the idea of the sharpness of someone(‘s mind).

I’m not sure I’m being clear. But basically, metaphors function, for me, as

  1. this thing is like this other thing and here are those two things
  2. this thing being like this other thing implies a correlation, which consequently describes or adds meaning to the original thing

Does anyone else have to take a second step to process metaphors?