in designing these kinds of situations is to understand where you're relying on the economic part of the bargain -- as with the parents paying the teachers -- and when you're relying on the social part of the bargain, when you're really designing for generosity.
设计这样的情形的关键 在于 理解交易双方的经济行为部分， 你依靠什么-就像家长们支付老师工钱- 和什么时候你依靠交易双方的社会行为部分， 什么时候你真正为人们共享而设计。