Geometric solids are not among the easily understandable topics in geometry lessons. Particularly due to their three-dimensionality, learners often need to engage their spatial imagination. Content related to geometric solids is therefore closely linked to real-world contexts. Kerstin Sitter examines how knowledge about solids is generated in extracurricular learning environments and how it can be sustainably processed and reflected upon in the classroom. The level-based model by Pierre and Dina van Hiele serves as a reference point and guideline. In terms of supporting self-directed learning processes, another focus of the study is on capturing fundamental recording skills. Through the targeted use of basic actions, such as identifying and realizing, as well as the verbal actions of describing and justifying, the author achieves a sustainable formation of concepts, as the research results impressively demonstrate.