In this paper, we analyze an investigative situation proposed to a class of 5th graders in a primary school. The situation is based on the following task: In a sale with group rates on a sliding scale, the students must find the lowest possible purchase price for a given number of tickets. A study of students' arguments made it possible to identify a large number of rhetorical forms. However, it turned out that one of the intrinsic features of the situation restricted the teacher's possibilities of making didactical use of the students' forms of reasoning and led him to try to support students' learning with "didactical reasons" rather than with "reasons for knowing".