Students tend to see mathematical functions as static, mysterious objects that must be approached according to a set of rules and procedures laid out in mathematics textbooks (Clement 2001). Because learners so rarely experience functions as a meaning-making tool that matters to their understanding of the world, they miss critical opportunities to experience the significance of mathematics within and beyond school. By contrast, real-world inquiry in the mathematics classroom creates opportunities for learners to see mathematics as a dynamic system--and as a vital conduit to other STEM fields (NRC 2013). In this article, the authors describe a project-based unit that integrated functions and modeling into an exploration of circuitry. This unit of six 50-minute lessons was developed as part of a three-year study and was incorporated into the curriculum taught in eighteen secondary mathematics classrooms in a diverse suburban school district in the Midwest. The unit was implemented in courses from eighth-grade mathematics to precalculus, serving as a hands-on introduction to linear functions in some cases and an opportunity to revisit and deepen core function concepts in others.