This paper presents a case of collaboration with three teachers and a didactician on task development within a developmental research project based on ideas of inquiry and learning community. The teachers' goal was to utilise a spreadsheet to orchestrate the pupils' investigations and build a library of tasks for the classroom. The focus is on one teacher in the team and his development of the spreadsheet as a personal instrument. Both activity theory and instrumentation theory are used to analyse the case. The insight gained suggests that both approaches were needed to get the full picture of the development.