This study investigated the role of tools in supporting students to reason about even and odd numbers. Participants included Kindergarten, Grade 1, and Grade 2 students (ages 5-8) at two schools in the USA. Students took part in a cross-sectional early algebra intervention in which they were asked to generalize, represent, justify, and reason with mathematical structure and relationships. Interviews were conducted with students before, during, and after the year-long intervention to explore the ways in which tools mediated their engagement in these practices in the context of questions about parity. In addition to cubes and "dot cards," which were introduced during the interviews, we found that students also used their fingers and hands as tools. We outline the affordances and constraints of these tools in supporting students' reasoning about the properties of even and odd numbers.