In the 1990s Steve Mann developed a number of user-interface strategies using natural interaction with the real world as an alternative to a command-line interface (CLI) or graphical user interface (GUI). Mann referred to this work as "Natural User Interfaces", "Direct User Interfaces", and "Metaphor-Free Computing".[2] Mann's EyeTap technology typically embodies an example of a natural user interface. Mann's use of the word "Natural" refers to both action that comes naturally to human users, as well as the use of nature itself, i.e. physics (Natural Philosophy), and the natural environment. A good example of an NUI in both these senses is the hydraulophone, especially when it is used as an input device, in which touching a natural element (water) becomes a way of inputting data. More generally, a class of musical instruments called "physiphones", so-named from the Greek words "physika", "physikos" (nature) and "phone" (sound) have also been proposed as "Nature-based user interfaces".[3]