This presentation is related to the praxis-developmental collaborative research project“Farstaprojektet” conducted 2004-2006. The project was developed as a collaborative projectwith teachers, educational researchers and mathematical researchers. The aim for the projectwas to explore the meaning of the goals to strive for (~subject specific qualities of knowing)stated in the National mathematic syllabus, in order to design and try out experimentalteaching related to these goals, i.e. a capability to solve mathemati-cal problems. The dataused in this presentation was produced during the second project year. The content chosen wasequation, since the teachers experienced that students had difficulties to master equations asa method in problem solving assignments. The collabo-rative work was divided into to parts.Firstly, we analysed mathematical textbooks in or-der to explore which assignments actuallyrequested equation as a method for solving them. The result was that most assignments wereeasier to solve using other methods. Secondly, the collaborative work was directed towardsdesign of experimental teaching. In this work we drew upon Davydov’s ideas of Developmentalteaching. In relation to this we aimed at developing assignments that, in their construction,require these capabilities here a confidant use of equations. This type of assignments wenamed key-assignments or epistemic assignments. In this paper we describe teachers’ use ofone assignment, which, according to our judgement, had the potential of becoming a keyassignment.A conclusion is that assignments cannot be designed as epistemic assignments;instead they become (or will not become) epistemic as a consequence of the teaching practice.