A metaphor is not exactly true. A metaphor is made up of two parts, a tenor, which is the subject of the metaphor, and the vehicle, which is the thing that illustrates the metaphor. You must find a simple metaphor to use. Metaphors are like similes in that they compare two different concepts, but they are not cued in language with "like" or "as" the way similes are.
Asked in Idioms, Cliches, and Slang, Metaphors Metonymy and Synecdoche Why do people say easy as cake when something was easy? The previous sentence is itself a metaphor, a figure of speech involving a comparison intended to communicate the idea that certain words make speech flavorful. It's meant to be understood as a figure of speech, not a factual statement. Metaphor is the seasoning of everyday language. Let’s take a closer look at this prominent branch of the figurative language family tree with some metaphor examples. There are more than a dozen different kinds of metaphors, including absolute, complex, conceptual, conventional, creative, dead, extended, mixed, primary, root, simile, submerged, therapeutic, and visual, that are used in writing to illustrate or symbolize something … Identifying metaphors requires an understanding of cultural language conventions, basic sentence structure and the communicative intent of the speaker. A metaphor, on the other hand, says that something is something else; that is, the girl's tears are equal to a river.
So, while you’re searching for the perfect metaphor to access the full depth of someone’s prior knowledge, put your ideas down on paper — the old-fashioned way. Metaphors are members of the figurative language family. In this case, the simile tells the reader that the tears are similar to a river, but not the same.