Obviously, not every man can marry his first-choice woman, and not every woman can marry her first-choice man. Life sucks that way. So, let's try for something weaker. Given a way of pairing off the men and women, say that it's stable if no man and woman who aren't married to each other both prefer each other to their spouses. In other words, you might despise your husband, but no man who you like better than him likes you better than his wife, so you have no incentive to leave. This is the, um, desirable property that we call "stability". -- Scott Aaronson "Quantum Computing Since Democritus"