In this study we have followed a participatory design process in a class of children aged 11 and 12. The development team, a group of Danish schoolteachers, invited the children to participate in the design of a computer game for mathematics education. The objective of the participatory design process was to have the children create a game close to their own interests, experiences and fantasies, hereby insuring that they would find the game interesting enough to play it in their spare time away from school. Prior to the design workshops, the development team had a discussion with one of their classes, and decided on a game of exploration where the player travels through time and space, and the purpose of the design process described in this paper was to develop this idea further. During this process it became clear that the teachers’ ideas in some sense differed from the children’s. In the teachers’ original concept, the landscape would represent the history of mathematics (e.g....