A metaphor is used to liken an object, person, or situation to another that may be completely unlike the former, but has one similar quality. Though there may be no obvious relation say between an owl and a human being, a metaphor creates this relation by connecting the two with one similar quality, i.e. the ability to stay awake at night. As such, a person who can stay awake through the night is called an owl. Reference http://www.buzzle.com/articles/metonymy-vs-metaphor.html