This article investigates the mechanisms that govern the processes of inclusion and exclusion of knowledge. It draws on three cases from Swedish classrooms about how energy is created as an area of knowledge. We are interested in how knowledge is made valid and legitimate in a school context, and in defining and finding tools to identify structures that govern potential meanings in a certain situation. To do this we develop a theoretical model that explains the preconditions for meaning. The purpose is to understand why certain knowledge is legitimated in the classroom and to explain how this happens. The analysis is based on participatory observations in classrooms, audio recordings of students engaged in group projects, educational materials, and the students' own work. The apparatuses of the school offer a wide range of possible meanings concerning energy. At the same time there are forces evolved in the school practice that effectively sift out what counts as values from what counts as facts and valid knowledge. These forces create a certain order and certain effects for what counts as truth. The article investigates the nature of the correlations between the different preconditions identified that makes one discourse more likely and "true" than another.