Because in "Pay it Forward", they capture this very idea of human networks as exponential functions.

电影“让爱传出去，深刻地描述了,人类网络是指数函数的观点。

哈佛公开课 - 幸福课课程节选

Pay it forward is a movement in LA, Mrs. McKenney.

把爱传出去“是洛杉矶兴起的一场运动。

哈佛公开课 - 幸福课课程节选