The authors had the chance to observe a class of 6th grade students in an elementary school in Mexico City. During the lesson observed, which was also video-taped and transcribed the notion of speed was introduced to the children. The teacher, well-experienced, did not follow the pedagogical prescriptions of collaborative work suggested by the text book presently in use, but rather applied a discursive teaching method in the mode of Socrates’Maieutics. Her main idea was to induce her students to use the ratios that arise from distance divided by time. However during the lesson, difficulties arose in the understanding and application of those ratios. One outstanding student, Marina, proposed a proportionality chart that the teacher initially deemed to be false, rejecting the proposal and failing to give it a place in her teaching process. The article analyzes the tangible parallel between the lesson observed and Socrates’interview of the slave in Plato’s Menon, and highlights a few exceptions that are interesting in view of their didactic significance.