The project Prim-E-Proof aims to generate substantial learning environments to support proving skills at the primary level. The learning environment “The Pythagoreans” on the theorem “the sum of two odd numbers is always even” is constantly further developed through design science research. The evaluation showed that the majority of the primary school pupils use empirical justifications and that the solutions of younger primary school children do not seem to differ much from those of older ones. A first concept for the reflection of justifications is supposed to support learners in developing valid justifications and teachers in implementing proof in primary school.