Card sorting has the potential to provide opportunities for exploration of a variety of topics and levels. In a card-sorting task, each participant is presented with a set of cards--each of which depicts a relationship--and is asked to sort the cards into categories that make sense to him or her. The concept of function is critical to students' future mathematical opportunities and as such is a central theme in the K-12 curriculum (NCTM 2000, 2006, 2009). Although card-sorting tasks have traditionally been used to assess teachers' content knowledge (see, e.g., Lloyd and Wilson 1998; Stein, Baxter, and Leinhardt 1990; Wilson 1994), the authors of this article argue that they can also provide powerful learning opportunities not only for teachers but also for students. The article describes the planning and facilitation of a card-sorting task by drawing on work in leading professional development sessions conducted with diverse groups of practicing teachers of grades 7-12 from local school districts. Although this task was designed for use with teachers, the authors invite readers to consider how it might be used with learners such as high school students.