. . .
A metaphor is a language device that invokes an image or concept through a familiar reference that is figuratively but not literally relevant to the subject at hand. (Clearly, metaphors are easier to define through the use of metaphor than by abstract definition.) When you pop someone's bubble, see the light at the end of the tunnel, find yourself up the creek without a paddle, have too many irons in the fire, or burn your bridges, you are using some of the vast wealth of metaphor that has become an established part of English vocabulary. Metaphors are so integrated into our language that we often give little thought to their origin or even to the fact that they are metaphors. When, for example, in a Western or other period film the need for haste is conveyed by telling a character to "step on it," few viewers recognize the anachronism in a nineteenth century character using a twentieth century metaphor inspired by the automobile. Commonplace metaphors such as these convey meaning clearly and obviously but are so overused that they add little poetry or imagination to the narrative and have become clichés to be avoided. Original metaphors, however, present one of the most fertile corners of the language for the seeds of creativity. Verbs, nouns, adjectives, and adverbs may all be metaphors to add variety, color, and elaborate imagery to a description: "She wilted dramatically at the grating voice of the choked garbage disposal."
A metaphor becomes mixed when the initial subject of the reference shifts. Mixed metaphors can be very descriptive and occasionally extremely apt, particularly when used in dialog as a character note. They do present problems however, frequently distracting the reader from the subject presented to the language used to present it. One must stop to collect one's thoughts for example, when someone says: I can see the carrot at the end of the tunnel or you can get into hot water skating on thin ice. Such utterances cause one's minds eye to stumble head over heels. Not to put too fine a point on it, nor to belabor that point, but changing horses in mid metaphor does have the effect of placing a hurdle in the stream of ideas and should be used only when that type of lurching conceptual disorientation is desirable.
I think we have misunderestimated the can of worms we threw away with the bathwater.
. . .
The simile is just a metaphor made obvious with the use of "like" or "as." His tie was like a comatose basilisk or Douglas Adams' wonderful inverted simile: The spaceships hung in the air just like bricks don't. Strictly speaking, in formal English "like" should be used when comparing nouns and "as" when comparing verbs. Thus the simile referring to how objects hang in the air (or don't, as the case may be) should be: The spaceships hung in the air just as bricks don't, but here one must defer to Adams who no doubt considered both options before choosing the less formal "like."
Excerpted with permission from SAT Practice: The New Verbal Section.