Lumps in the Batter of Mixed Metaphors

This entry will actually cover three topics: defining metaphor, mixed metaphors, and dormant metaphors.
Before discussing the use and abuse of metaphors, it is worth clarifying the definition of metaphor. A metaphor is a figure of speech that directly calls one thing something else, or says that it is that other thing. In grade-school our teachers did a great job of helping us distinguish between simile and metaphor, the former using like or as and being explicit, the latter being implicit. Unfortunately, analogies are not contrasted with metaphors enough.
An analogy is a similarity (or a comparison based on a similarity) between two things that are otherwise dissimilar. While similes, metaphors and analogies all extend meaning through correspondence, typically metaphors and similes give elegant depth to a description, while analogies give practical understanding. This is why analogies are a central tool in teaching and explaining.
Yet too frequently people incorrectly call analogies metaphors. Usually these infractions begin with the phrase “A good metaphor for … is …”.
For example, “A good metaphor for the stages of intimacy with someone is rounding the bases to score a run in baseball.” This is actually an analogy, and an overworked and generally lame one at that. Try to fill the … in the phrase with an example of a metaphor. Semantically this construction is perilous. A safer approach is to start “A good metaphor is …”. Just say the metaphor—then describe it.

Unless being used for purposeful humor (like the title of this posting), mixed metaphors are embarrassing mistakes that leave readers either confused or laughing at (instead of with) the writer. One of my favorites is in a speech by a scientist who referred to “a virgin field pregnant with possibilities.”
Clichés can also spin out of control: “They’ll be watching everything you do with fine-toothed comb.” Sorry, combs can’t watch anything.
A truly amazing foul up is when two expressions are mixed together incorrectly and the result takes on a whole new (and unintended) meaning beyond the confused parts. The website http://www.stuntmonkey.com/metaphor/ calls this a triple wammy. The following is my favorite: “We’re starting from ground zero.” This confuses “starting from square-one/the beginning/zero”, with “building from the ground-up,” to result in dealing with a nuclear attack. Remarkable!

Finally, a dormant metaphor is somewhat like a Freudian slip. It occurs when the literal meaning of the metaphorical language infringes on its extended purpose. This comes from poor contextual placement. For instance, consider this sentence taken from a law journal: “This note examines the doctrine set forth in Roe v. Wade and its progeny.” Since progeny literally means offspring, this is probably not the most appropriate place to use it more loosely referring to a result or product.
On a subtler note, the vehicle (i.e. literal sense of the metaphorical language) should harmonize with the tenor (i.e. intended metaphorical sense) of a metaphor. For instance, “the internet superhighway” makes sense because both highways and the internet are modern technological achievements. “The internet super-trail” falls flat.
However, sometimes the discord is purposeful and effective, as in “concrete jungle”.

Please send in comments with some of your own favorite uses and abuses of metaphors.