In working with the youngest learners, drawing from their own lived experiences to inform mathematics instruction is a critical piece for engaging with as well as supporting students in mathematizing their world (NAEYC and NCTM 2002, 2010; NCTM 2020). In the interaction described in this article, four-year-old children used mathematics to examine the location of and access to public playgrounds in their community. They collected and recorded data, using the information to advocate for a social justice issue.