Think of how stingy you are with your money: if someone on the street asks for $10, you're not giving it to them. There's no way you're giving a 30% tip for a normal dining experience. And so on.
想想你在花钱上是多么的吝啬,如果有人在街上问你要10美元,你是不会给的。普通的一顿饭要收你30%的小费,那也是没门。诸如此类的事情还有不少。