One approach adopted in the design of graphical user interfaces is to create icons and dialogue structures, and to consider the tasks supported by interactive systems, according to a consistent theme. By making the theme some aspect of the real world, or a task domain with which the user is assumed to be familiar, it is said that some metaphor or analogy has been adopted in the design of the user interface's model world. It is also suggested that by adopting an analogy in the appearance and behaviour of the on-screen objects that make up the model world, users' existing knowledge can be carried over and the novel interactive system will be easier to learn and use than if new knowledge structures have to be acquired.