" 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. "

― Mark Forsyth , The Elements of Eloquence: How to Turn the Perfect English Phrase