||In this report we indicate a research related to learning the geometry of solids, that shows some examples of families of solids that students include in the mental objects they construct for these families and some mistakes they make as a consequence of the difficulties that mathematical content (concepts, properties, relations, etc. ) present for the students. This can be prompted o encouraged by the teaching of the subject. Results here reported are obtained through the analyses of information requested from interviews and working sessions carried out with two groups of students aged 12 years. We also analyze the answers that Teacher Training College students gave in writing to some activities they were asked to do before being treated in class.