Problem solving tasks offer valuable opportunities to strengthen prospective elementary teachers' knowledge of and disposition toward mathematics, providing them with new experiences doing mathematics. Mathematics educators can influence future instruction by modeling effective pedagogical strategies that engage students in making sense of processes of mathematical reasoning. What follows is a description of a well-designed task and the role played by one mathematics educator in engaging prospective teachers in processes of mathematical reasoning. It is hoped that this description can shed light on some general ideas about the design and implementation of problem solving tasks in courses for future elementary teachers. (Contains 2 figures.)