An important aspect of research on the use of digital games in education addresses the kinds of learning processes generated through students designing and constructing games as well as playing them. The process of design and construction has been pedagogically engineered so that students engage in collaborative learning and in generating meanings around the rules of the game. Here we report on the preliminary results of a study investigating how students discuss, negotiate and learn rules which embody the concept of "sustainable lifestyles". We used a digital authoring system for the development of Sim-City like games around the idea of sustainability (we call it the 'Sus-X' system). These games operated as 'microworlds' and at the same time as negotiable and improvable boundary objects between students and teachers. The students were first given a digital game we developed for them called "PerfectVille". They were asked to play the game, discuss and question the rules and then create their own game ("MySusCity") by changing the rules of the former so that it portrayed more sustainable lifestyles. Playing these kind of games involves the students visiting as many places as possible in the imaginary city and being careful not to run out of resources, such as money, energy, etc. Sus-X games are different to Sim-City games in that their functioning mechanism is built upon fallible "sustainability" axioms. The "PerfectVille" game, which was a pedagogical artefact where the underlying sustainability axioms were to be questioned and changed by the students - and in that sense, a boundary object -, indicates a hectic way of life, since the winner is the one who manages to visit as many different places as possible within a restricted amount of time. At the end of the game, by discussing their scores the students were led with the teachers' help to challenge the "sustainability model" which is inherent in the "PerfectVille" game, and to introduce an alternative "model" to be integrated in the new game which they were going to design. Preliminary research results on the game play that has been conducted with two groups of environmental education teachers indicate that the discussion around game play and game design progressively focused on sustainablity issues bound with game strategy in an integrated way. The mediation of the teachers - researchers was necessary to help discriminate, identify and focus on the essence of the "sustainability" concept and the complexities of sustainability issues. The multiple role of the student teachers, as game players and designers, seemed to generate a significant degree of engagement and collaborative activity.