In an old manuscript, I once wrote: He was a mad hen, flailing his arms. An expensive editor noted: Since hens don't have arms, your imagery won't work. Imagery is good, but it has to make sense. You could say, "... flailing his arms as if he were an angry hen chasing chicks with flapping wings." 

After all these years, I still giggle at that. To me, her suggestion seemed both nonsensical (anyone who's ever seen a mad hen knows good and well those flapping wings are their arms) and clunky (why not go ahead and take that line to its logical limit? "...flailing his arms as if he were an angry hen chasing chicks with flapping wings that ran around the barn like chickens with their heads cut off."). But seriously, what I took away from that experience, and the only thing that really mattered, was that I needed to work on my metaphors. 

So what is a metaphor? It's a comparison. It states that one thing is something else, although we know the thing really isn't.

Love is a rose. 
Aunt Fran is a mule. 
The office is a sauna.

Metaphorically speaking, metaphors are a mental slideshow. Whenever the eye spots one, the brain flashes a recognizable snapshot onto the internal screen of the mind and communication is established. Ah, yes. We see a rose with its thorns and we understand that love can be both beautiful and painful. We see a kicking mule and we understand that Fran is stubborn. We see a steamy sauna and we understand the office is hot. The something represents the something else. 

Sounds simple enough, but the application of this writing tool can still be tricky. A good one makes us smile. A bad one makes us groan. 

WHAT MAKES A BAD METAPHOR? 

Clichés. Tired, familiar expressions were once brand new cars that have been depreciating since they rolled off the tongue. They should be garaged. The best way to spot a cliché is to look for comparisons that required absolutely no thought. Love is a rose. Aunt Fran is a mule. The office is a sauna. Etc.

Mixers. Using more than one metaphor at a time will always elicit a dreaded, Huh? If a particular character is a snake that slithers into a woman's bed, he can't hop out of it the next morning and fly away. Unless he’s some kind of freak snake that’s just come out of a nuclear plant. When we cause our readers to scratch their heads, our metaphor has been rendered useless. Pick one train of thought and follow it through to the end.

Fads. Pop culture not only dates our work, it risks ostracizing readers who don’t keep up with it. Some items are more obscure than others, obviously, but a few folks in the world don’t even know who Britney Spears is, let alone that she recently shaved her head. Any comparison to her would fly right past them. Avoid what’s currently popular and use timeless, universal comparisons. 

Overkills. The beauty of a metaphor is its rarity. If one pops up in almost every paragraph, it soon becomes an obnoxious finger tapping the shoulder. Look at me, I’m being literary! Issue a metaphor as you would a compliment—honestly and sparingly.

WHAT MAKES A GOOD METAPHOR?

Originality.  No two writers see things in the same way. Embrace your singular perspective. The next time you seek a metaphor, pause your fingers over the keyboard and ask yourself, Of what does this character or situation remind me? Allow your brain time to rummage through your internal slideshow and create the perfect snapshot. It will help your readers see on a much deeper level. And maybe, for a moment, a knowing smile will spread across their faces. If so, the metaphor has done its job.

IN A NUTSHELL
A metaphor is a dollop of wisdom on a slice of life.