The aim of this paper is to describe a methodology for using Cultural-Historical Activity Theory (CHAT) at the initial stages of the design process of an educational game, by exploring how the theory can be used as a framework for producing not only usable but also useful computer tools. The research also aimed to investigate how the theory could be used for designing computer tools for learning science. Although CHAT and specifically the concepts of "activity system" and "contradictions" were used as the basis for the design and development of the educational game, subsidiary design guidelines also contributed significantly to this process. These derived from the research fields of Human Computer Interaction and Science Education and from students' everyday experiences when playing video games at home. The educational game produced was concerned with the teaching and learning of "Expansion and Contraction of Air" in primary science, a subject that existing research suggests is conceptually difficult for students. As far as the main outcomes of the study are concerned, it was revealed through the implementation of the game that contradictions could be adequately resolved, while stakeholders' needs and motives could be appropriately addressed.