This paper describes the ‘Planet Game’ case study, also called the ‘Astronomy Game’, initially proposed to the participants of the workshop “Comparing Educational Modelling Languages on a Case Study” which was held in Heerlen, The Netherlands, during ICALT 2006. This case study is supposed to facilitate the comparison of approaches (model/tool/…). This is only a « framework » that has been sometimes adapted by the authors of the papers of the special issue. This paper gives also guidelines to structure the paper, again to improve the comparison of the approaches for the reader of the special issue.