In user interface design, an interface metaphor is a set of user interface visuals, actions and procedures that exploit specific knowledge that users already have of other domains. The purpose of the interface metaphor is to give the user instantaneous knowledge about how to interact with the user interface. They are designed to be similar to physical entities but also have their own properties (e.g., desktop metaphor and web portals). They can be based on an activity, an object (skeuomorph), or a combination of both and work with users' familiar knowledge to help them understand 'the unfamiliar', and placed in the terms so the user may better understand.