Have you ever seen someone eating really sloppily and said, 'He's such a pig'? Of course you don't mean that person is literally a pig. What you're using here is a metaphor. A metaphor is a form of figurative language where one thing is said to be the same as another thing. In that example, a sloppy eater is said to be the same as a pig. Metaphors are often used to invoke imagery, and to give the listener a clear idea of what you mean in a short sentence. Metaphors are also very common as a literary device. One place we can see examples of this is in Erich Maria Remarque's novel, All Quiet on the Western Front
