This paper investigates conceptions of mathematical investigation and proof in upper-secondary students. The focus of the paper is an intervention that scaffolds the interaction between open explorative activities and the development of proof sketches through explorations of lattice polygons, aiming at proving Pick’s theorem. In the process we investigate whether and how the conceptions of proofs and explanations in mathematics change. We work with the hypothesis that the problem of supporting the transition to deductive proofs in upper-secondary school students can at least partly be explained as a problem of bringing their empirical investigations into the deductive proof process in relevant and productive ways. Through our analyses of the
portfolios and deliberations of the students, we are able to assess their performance of proofs and the conceptions of mathematical methodology before and after the intervention.