if your friend asked to borrow $100 from you, ... you would probably not say-- you might actually put interest on this person, so you could say, alright, I'll do it with 5% interest, pay me back in a year.

你的朋友向你借100美元，你可能不会说-,你可能会对他收取利息，因此你没准会说，好吧，我要收5%的利息，一年之后连本带利还我。

耶鲁公开课 - 金融市场课程节选