When writers transfer the characteristics of one thing to the reality of another, they are using a metaphor. To create a successful metaphor, they chose carefully so that their readers’ associations connect to the intended meaning.
For example: “An avalanche of homework” refers to the imminent collapse of a large volume and to the risk of becoming “snowed under.” Compare that, then, to a “mountain” of homework. In the former, tragic consequences are avoided by triggering a series of smaller collapses; in the latter, determined climbing will scale the peak successfully. A mountain seems difficult, but in an avalanche, the risk of being buried alive is ever-present.
A few years ago, I stepped outside my usual composition gig and taught the summer session of a freshman botany class. Once past the leaves-and-flowers discussions, we needed a way to think about cellular components and chemistries too small to see with the eyes, so we workshopped with each other for appropriate comparisons. One group decided that DNA translation is a blinking neon Las Vegas sign: the bulbs of the sign’s perimeter light one after another until the outline is complete, they blink, then the text in the center lights up, too. Once everything is lit, every bulb flashes off and the process starts again, looping endlessly. It’s an unusual metaphor and more importantly, an effective one. No one in the class missed the DNA translation questions at test time, which means the group that proposed the metaphor was able to connect with their audience.
When skillfully chosen, metaphors can distill an entire story without disrupting the flow of larger text. In the Arthur Conan Doyle short story “A Study in Scarlet,” Dr. Watson, who has not yet met Sherlock Holmes, has “naturally gravitated” to “London, that great cesspool into which all the loungers and idlers of the Empire are irresistibly drained.” To understand the metaphor, you must know that a cesspool is a pit that sewage drains into for settling. Doyle implies loungers and idlers are no better than human waste; however, he also makes a deeper, darker association between London’s human waste and diseases such as cholera and the Bubonic Plague.
Disease infection rates for pre-20th century Londoners were exacerbated by the population density of the city and the poor handling of waste disposal. The loungers and idlers, then, could potentially “infect” the entire population of London. Does it matter if the darker connection to disease transmission is made by the reader? Probably not since the cesspool association is enough to influence a reader’s perception of Victorian London as a place to live, but can you see how the association strengthens when the connection is puzzled out? Doyle counted on his fellow countrymen having a flash of recognition that London has a long, dangerous history of being a cesspool.
Nearly anything can become a metaphor for something else, assuming the context is appropriate. Check that your metaphor and your topic are not opposites in your readers’ minds, or that if they are, you are exploiting that dissonance. While the clever among you can probably make a case for comparing database software to a cotton gin, those two worlds don’t overlap well.
The one absolute: don’t use metaphors that are so old they have lost their power (e.g., whip smart, clean slate, basket case), or that have become too familiar to be noticed as metaphors (e.g., dead weight, home run). Writers use clichèd metaphors like these because they easily spring to mind. Put some thought into your choices for metaphors — charm, inform, or surprise your readers.