An analogy is a special form of comparison in which one part of the comparison is used simply to explain the other, as in the following example:

In like manner, geologists will sometimes use the calendar year as a unit to represent the time scale, and in such terms the Precambrian runs from New Year’s Day until well after Halloween. Dinosaurs appear in the middle of December and are gone the day after Christmas. The last ice sheet melts on December 31st at one minute before midnight, and the Roman Empire lasts five seconds. With your arms spread wide. . . to represent all time on earth, look at one hand with its line of life. The Cambrian begins in the wrist, and the Permian Extinction is at the outer end of the palm. All of the Cenozoic is in a fingerprint, and in a single stroke with a medium-grained nail file you could eradicate human history. Geologists live with the geologic scale. Individually, they may or may not be alarmed by the rate of exploitation of the things they discover, but, like the environmentalists, they use these repetitive analogies to place the human record in perspective—to see the Age of Reflection, the last few thousand years, as a small bright sparkle at the end of time.

—JOHN MCPHEE, Basin and Range

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Analogies are not limited to abstract, scientific concepts. Writers often use analogies to make nontechnical descriptions and explanations more vivid or to make an imaginative point of comparison that serves a larger argument. Consider the following example:

But now that government has largely withdrawn its “handouts” [to the welfare poor], now that the overwhelming majority of the poor are out there toiling in Wal-Mart or Wendy’s—well, what are we to think of them? Disapproval and condescension no longer apply, so what outlook makes sense?

The “working poor,” as they are approvingly termed, are in fact the major philanthropists of our society. They neglect their own children so that the children of others will be cared for; they live in substandard housing so that other homes will be shiny and perfect; they endure privation so that inflation will be low and stock prices high. To be a member of the working poor is to be an anonymous donor, a nameless benefactor, to everyone else. As Gail, one of my restaurant coworkers put it, “you give and you give.”

— BARBARA EHRENREICH, Nickel and Dimed

Analogies are tricky. They can be useful, but they are rarely consistently accurate at all major points of comparison. For example, in the preceding analogy, the working poor can be seen as philanthropists in the sense that they have “made a great sacrifice” but not in the sense that they are selflessly sharing their wealth. Analogies can powerfully bring home a point, but skilled writers use them with caution.

Nevertheless, you will run across analogies regularly; indeed, it would be hard to find a book without at least one. For abstract information and in certain writing situations, analogy is often the writing strategy of choice.