You don't really understand human nature,unless you know why a child on a merry-go-round will wave at his parents every time around and why his parents will always wave back.——William D.Tammeus 你不可能真正了解人性，除非能理解为什么旋转木马上的孩子会在每次转过父母时和他们招手，而父母也会每次和他们招手。