This paper explores the planned use of challenging mathematical tasks. These tasks provide the opportunity for students to improve mathematical thinking by working on problems that they do not yet know how to answer. This research involved a heterogeneous class of year 3 students from a Catholic parish primary school in the northern suburbs of Melbourne. A rubric was also developed to use in conjunction with these tasks, to support discussions with students, broaden their strategies in finding solutions and thereby improve their conceptual understanding. These pedagogical approaches were found to support the improvement of both students’ conceptual understanding in mathematics and teachers’ reflective practice.