In metaphor and simile you say that two things have a couple of qualities in common. It generally has to be at least two: one obvious one and one that is strongly implied. Suppose that a chap tells the girl he loves that her eyes are as green as emeralds: she’ll probably take that as a compliment, not because emeralds are green but because they’re valuable. If he tells the girl that her eyes are as green as mould, he’ll get a slap; not because he’s inaccurate but because it’s always the second, implied comparison that’s important.
