Idioms as isolated metaphors. This exercise is a variation on an earlier one in this unit. Each of the following sentences contains an italicized idiomatic expression that would make it anomalous if it were interpreted literally (i.e. compositionally). Briefly explain this anomaly for each sentence, and then describe what kind of intended non-literal meaning the sentence typically conveys. Finally, if possible, try to suggest what kind of isolated metaphor each sentence might be an example of. If it's not possible to formulate an appropriate metaphor in simple terms, try at least to explain to what extent the expression might be partially analysable (i.e. compositional). The first item, repeated from an earlier practice, is done for you.
rank is a snake in the grass
Anomaly: Frank is being equated with (or classified as) being a particular kind of animal (a snake), which is literally untrue.
Non-literal meaning: Something about Frank's behaviour is untrustworthy. Metaphor: PEOPLE ARE ANIMALS. (Note that this is not completely isolated, since we do have such expressions as Frank is a tiger, Mary is a lamb, etc.) In this case, Frank's apparent identification as a snake is meant to associate him with the popular idea that snakes are sneaky and underhanded.