This case study introduces students to the true story of Caster Semenya, an outstanding female mid-distance runner from South Africa.  Caster won the 800 m race at the 2009 Track and Field World Championships when she was just 18. Since then, she has faced scrutiny about her sex and gender. Due to her high levels of endogenous testosterone, she was banned from running for almost a year by the track and field governing body, the IAAF. In this case study, students use information from news reports and Caster's appearance to determine the possible biological underpinnings likely responsible for her anatomy and physiology.  Students are also asked to consider if using circulating testosterone levels to determine if a female can race is a valid method of exclusion. This case asks students to think about how sex and gender are related, and pushes them to discuss societal implications of labeling both of these constructs as binary when in fact they are much better described as spectra.