The computer is like a typewriter. The computer is like a filing cabinet. The computer is a personal servant ready to obey your every command. It is often claimed (e.g., Carroll and Thomas [3], Rumelhart and Norman [7]) that the best way to introduce a new user to a computer system is to draw an analogy between the computer and some situation familiar to the user. Given the analogy, the new user can draw upon his knowledge about the familiar situation in order to reason about the workings of the mysterious new computer system. For example, if the new user wants to understand about how the computer file system works, he need only think about how an office filing cabinet works and then carry over this same way of thinking to the computer file system.