When we render a button on the screen, we are making a contract with the user that that button will visually change when she pushes it: It will appear to be depressed when the mouse button is clicked over it.

• 当我们在屏幕上放置一个按钮时，就与用户建立了一个契约。 用户按下按钮，按钮在视觉上就会发生变化。 用鼠标单击，它会向下凹。