In this paper we describe a diagnostic study to investigate students' understanding of two basic formulae in physics. Based on the findings of the study, we have developed a classroom activity focused on the interpretation of formulae. The activity was developed cooperatively by physics education researchers and high-school physics teachers and was tried out in the teachers' classrooms. We describe the activity and present findings about students' attitudes towards the activity and the progress in students' understanding of three formulae. (Contains 3 tables and 2 figures.)