If you want two extremely selfish people to fairly share a cake, the best way is not to find a noble person to supervise, but four words: "You cut, I choose." Because once the cutter divides the cake unevenly, the larger piece will definitely be taken by the other person, so they will desperately try to cut the cake as evenly as possible. Without supervision and without moral constraints, both people can be selfish to the extreme, but the result will inevitably be fair. This seemingly simple trick actually reveals a deeper logic: true cleverness in fairness is not about suppressing human nature, but about designing rules that automatically guide human self-interest toward fair outcomes.