In this chapter we envision the science classroom as an authentic scientific community. In this vision, student ideas can influence the trajectory of scientific investigation. Teachers serve as experts and guides, but they also can learn alongside their students. To do this, they need to listen to the students and be able to build on students' original ideas to help them learn. What knowledge does a teacher draw on in such a classroom? In this chapter we empirically investigate some ways in which a teacher can utilize both knowledge of the subject matter and knowledge of science practices to respond productively to student thinking. We present data from a large study of knowledge for teaching energy. The subjects of this study were high school physics teachers. We found that in some instructional situations, teachers with insufficient content knowledge cannot productively respond to student reasoning. We also found cases where teachers can compensate for lack of content knowledge if they are skilled in science practices. To explain our findings, we hypothesize the existence of two types of content knowledge: foundational content knowledge and elaborative content knowledge. Furthermore, we suggest that foundational content knowledge along with knowledge of scientific practices can allow teachers to compensate for insufficient elaborative content knowledge. We discuss the implications of our hypothesis for future research and for the preparation and professional development of physics teachers. [For the complete volume, "Pedagogical Content Knowledge in STEM: Research to Practice. Advances in STEM Education," see ED613708.]