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"Because the way you grow old is kind of like an onion or the rings inside a tree trunk or like my little wooden dolls that fit one inside the other, each year inside the next one." This quotation shows examples of Simile.
This sentence is considered to be a simile because it is the figurative speech that makes a comparison of two different things in ordinary day to day communication activities. In SImile, the sentence includes the words "like" or "as."
When a writer uses a simile is because it wants the reader to compare one thing with another or increase the interest with the comparison.