The study of three-dimensional objects is undertaken in the Mexican curriculum throughout all levels of basic education. However, few studies relate to the mathematics knowledge needed to teach those ideas. This report represents an approach to the study of such knowledge. The approach is based on an account, developed from class observations, of three six grade teachers and their use of mathematics knowledge in teaching their students to calculate the volume of prisms, in two schools in Mexico City. The results provide evidence related to some sub-domains of mathematics knowledge for teaching. While there are some strengths related to this knowledge, teachers also showed that there is room for improvement. This speaks to the need to develop training spaces (both for preservice and in-service teachers) where the analysis and discussions stimulate learning of both the geometric and pedagogical knowledge needed for teaching practice. [Funding for this project was provided by Conacyt/SEP/SEB. This paper is presented in both English and Spanish. For the complete proceedings, see ED583608.]