Metaphor

As you probably already know, a metaphor compares one item with another, but it does so by direct substitution of nouns. X is Y. For example, if you say, "My brother is a pig," you are not, of course, saying that your brother has four legs, a curly tail, and plays in the mud. You are saying that your brother and pigs have qualities in common: they are messy creatures who lie around all day.

Again, you are substituting one noun (person, place, or thing) for another. Therefore, "My sister is fat" is not a metaphor because "fat" is not a noun. "Fat" is not a person, place, or thing. However, "My sister is a whale" is a metaphor because "whale" is a noun.

So, if you struggle generating metaphors, generate a simile first, then delete the "like" or "as."