Most people, school teachers and children included, are altogether too sure about what gender is: there are two "opposite" sexes, man and woman, and gender is the inevitable categorical expression of natural sex. Like all commonsense views, however, the gender binary has been socially constructed through normalising discourses that frame certain ways of thinking and doing as "commonsense" and thus unassailable. As Judith Butler points out, resisting these constructed norms requires a conscious effort to deconstruct our understandings, to "lose our expertise". Drawing upon data collected as part of a broader study addressing lesbian, gay, bisexual and transgender equalities in English primary schools, this analysis involves a deep interrogation of sex and gender with the aim to help teachers and children understand how sex/gender categories are policed and to recognise and question our complicity in maintaining commonsense understandings of sex and gender. (Contains 6 notes.)