In this paper, we analyse an activity for 1st grade students, taken from the official pedagogical material for mathematics in French-speaking Switzerland. This activity is part of the curriculum about addition and comes in the form of a dice game. After some succinct considerations about games in mathematics education, we give an a priori analysis (according Brousseau's theory of didactic situations) of the activity. We then give account of an experimentation we made in Geneva, first with the teacher in her class and then with two duos of students outside the class. Finally, we suggest some modification in the didactical design in order to make this activity more pertinent.