A new pervasive digital game is discussed, relating technical and conceptual innovation. A combination of sensor technologies enables a responsive visual and auditory environment to be overlaid on the real world. This allows processes within the player's body to be mapped to the environment through which the player passes, externalising the internal. Rather than using technology to replicate the rigid goals and structures of many conventional games, this game explores the concept of 'open play', a form of personal exploration. The work is an interdisciplinary collaboration between digital artists and health scientists with an agenda to alter players'attitudes to the body and health as well as to break new ground artistically.