Recently, there have been increased calls for enhancing preschool children's mathematics knowledge along with increasing calls for preparing preschool teachers to meet the demands of new preschool mathematics curricula. This article describes the professional development program Starting Right: Mathematics in Preschools. Focusing on four episodes that revolve around the topic of equivalent sets, it demonstrates how engaging preschool teachers with challenging tasks may promote their subject matter knowledge as well as their pedagogical content knowledge, leading to an interweaving of knowledge and practice. Assessment of the program included assessing the children's knowledge. Results indicated that students of participating teachers learned to be more flexible and to give more possible answers than students of non-participating teachers.