This paper explores what happens in the encounters between presentations of "basic facts" about the human genitals and 15-year-old students during a biology lesson in a Swedish secondary school. In this paper, meaning making was approached as relational, context-dependent and continually transacted. For this reason the analysis was conducted through a series of close readings of situations where students interacted with each other and the teacher in opening up gaps about alternative ways of discussing gender. Drawing on Foucault's theories about the inclusion and exclusion of knowledge and the subsequent work of Butler and other feminist researchers, the paper illuminates what gendered relations remain tacit in the conversation. It then illustrates possible ways in which these tacit gendered meanings could be made overt and discussed with the students when making meaning about the human genitals. The paper also shows how the ways in which human genitals are transacted in the science classroom have importance for what kind of learning is made available to the students.