About 80 middle and secondary school teachers participated in the professional development sequence as a part of a larger professional development project. Sessions provided participants with half-day experiences in an inquiry-oriented classroom taught by a master teacher, in which they experienced examples of inquiry lessons as both student and teacher. The remainder of the day consisted of content-rich activities that explored mathematics and pedagogy in depth and sought to reveal and develop teachers' mathematical understanding and practices. The authors developed a sequence of professional development activities that sought to reconnect teachers with their own mathematical ways of thinking, namely of reasoning using definition. The authors used the van Hiele levels of geometric thought to select and sequence activities and as a lens through which to view teacher work. In this article, they present descriptions of the activities and a rationale for their selection along with insight gained from teachers who completed the activities.