Calvin and Hobbes: "A good compromise leaves everybody mad."

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I say something like this to my kids when I'm tired of mediating and I want them to solve their own disagreements. "My definition of success is that you are both crying after I've made my decision."

(for the record, I did not have this policy when they were toddlers, only as tweens; I'm not a monster)

Such policies encourage bad behaviour - being reasonable makes it guaranteed you loose against unreasonable.