Videogames are interactive by nature - people proceed in games by doing things, and this experiential quality lies at the very core of game design. Without interaction, it isn't a game. Videogames are popular precisely because of the experience - games designed for learning can do no less. However, to be feasible for use in formal educational settings, they must do more, and while we are making progress studying games in classrooms, there remain few structured approaches to analysing games that do not include classroom testing. This paper outlines the author's Four Pillars of Game-Based Learning (4PEG) which can be used to perform a structured analysis of both COTS and serious games to assess whether or not a game has potential for use in the classroom.