A powerful way to build students' mathematical power and agency is through Teaching Mathematics for Social Justice (TMfSJ), where students read and write the world with mathematics (Gutstein, 2005). Reading the world with mathematics uses mathematics to understand relations of power, resource inequities, and disparate opportunities between different social groups and to understand explicit discrimination based on race, class, gender, language, and other differences, while writing the world with mathematics involves taking action (Gutstein & Peterson, 2013). In this article, the authors share one teacher's story of how she used her students' interests to uncover reasons for the economic inequalities and fair access to sports at their local high school.