gestures, or mouse gestures sketched with a computer mouse or a stylus. Graphical user interfaces (GUI) accept input via devices such as a computer keyboard and mouse and provide articulated graphical output on the computer monitor. There are at least two different principles widely used in GUI design: Object-oriented user interfaces (OOUIs) and application-oriented interfaces. Hardware interfaces are the physical, spatial interfaces found on products in the real world from toasters, to car dashboards, to airplane cockpits. They are generally a mixture of knobs, buttons, sliders, switches, and touchscreens. provide input to electronic or electro-mechanical devices by passing a finger through reproduced holographic images of what would otherwise be tactile controls of those devices, floating freely in the air, detected by a wave source and without tactile interaction. Intelligent user interfaces are human–machine interfaces that aim to improve the efficiency, effectiveness, and naturalness of human–machine interaction by representing, reasoning, and acting on models of the user, domain, task, discourse, and media (e.g., graphics, natural language, gesture). Motion tracking interfaces monitor the user's body motions and translate them into commands, currently being developed by Apple. Multi-screen interfaces, employ multiple displays to provide a more flexible interaction. This is often employed in computer game interaction in both the commercial arcades and more recently the handheld markets. Natural-language interfaces are used for search engines and on webpages. User types in a question and waits for a response. Non-command user interfaces, which observe the user to infer their needs and intentions, without requiring that they formulate explicit commands. Object-oriented user interfaces (OOUI) are based on object-oriented programming metaphors, allowing users to manipulate simulated objects and their properties. Permission-driven user interfaces show or conceal menu options or functions depending on the user's level of permissions. The system is intended to improve the user experience by removing items that are unavailable to the user. A user who sees functions that are unavailable for use may become frustrated. It also provides an enhancement to security by hiding functional items from unauthorized persons. Reflexive user interfaces where the users control and redefine the entire system via the user interface alone, for instance to change its command verbs. Typically, this is only possible with very rich graphic user interfaces. Search interface is how the search box of a site is displayed, as well as the visual representation of the search results. Tangible user interfaces, which place a greater emphasis