This text addresses the problem of the discrepancy between teachers’ and students’ positions in negotiations about the authenticity and legitimacy of school science activities. The study focuses on the apparent conflicts concerning legitimacy and authenticity when teachers and students bring attention to safety, authenticity and accuracy during issues laboratory activities. The analysed data are excerpts made from video observations in two science classes. Analysis was made using epistemological moves describing how teachers and students make their activities relevant. The result indicates that in the classroom conversation about laboratory practice, teachers sometimes draw the attention to safety, procedures and accuracy to legitimize the activity and how they try to control it. Negotiations concerning the legitimacy and authenticity of activities seem inevitable. Unless understandable agreements are reached, the negotiations jeopardize a successful understanding of the Nature of Science (NOS). Misunderstanding of the authenticity of activities contributes to a reduction of their legitimacy, and undermining teaching of context independent knowledge.