To be effective, physics teachers need both content knowledge and pedagogical content knowledge, which includes knowledge of student conceptions and effective teaching strategies. Although much information is available on student conceptual and reasoning difficulties in physics, much less information is available on how to remedy such difficulties. In this paper I describe a teaching strategy, concept substitution, which is useful when student difficulties arise from a failure to distinguish distinct but related physics concepts. By using the topic of electric circuits as the context, I show how this strategy enables the teacher to identify and build on students' correct intuition, while enabling students to distinguish among related concepts. I also illustrate the complexity of the conceptual change process, including the presence of intermediate conceptions while the process is taking place.