These mental models are active, called into play to explain the world and to predict which actions to take. Mental models are incomplete and often inaccurate, but they help people deal with the world on a daily basis. Users develop mental models for computer systems, and HCI researchers believe that the interface is the basis for the mental models that users develop. Designers are thus concerned with ways to assist users in quickly developing accurate and useful mental models for their systems. A common approach is to define a metaphor that links existing user knowledge to system function.
Metaphors are useful, but they can also constrain the user's view of the unique aspects of a system.
In effect, concern with ease of learning can eventually interfere with skilled use. The most general solution is to develop interfaces that adapt to the users' abilities and needs. In addition to the many technical challenges such interfaces offer, there is a philosophical debate over whether the interface should adapt to the user automatically or only through specific user control. The physical components of the interface should be ergonomically designed, taking into account the comfort and health of the user as well as his or her special needs and characteristics. For example, a touch panel design for a word processing program demands far too much arm movement for lengthy sessions, but serves quite nicely in an information kiosk of a shopping mall when positioned to be touchable by users of various heights.