This article examines the way student teachers make sense of conceptual tools when writing cases. In order to understand the problem-solving process, an analysis of the interactions is conducted. The findings show that transforming practical experiences into theoretical reflection is not a straightforward matter. To be able to elaborate on the task it is crucial to make meaning of the tools. It is demonstrated that the institutional practices, rules and expectations must be explicit for the students. The overall aim is to demonstrate that activity theory can enhance our understanding of the interdependence between activity, task and mediating tools. (Contains 4 figures.)