In this paper, we begin to explore the role of content knowledge in responsive teaching (RT), using "in situ" data to draw out and speak to a latent disagreement within the literature. We claim that one role that content knowledge plays in RT is to support teachers in eliciting, seeing, and then pursuing disciplinary connections within their students' thinking. We suggest an approach to teacher education that draws on the historical wisdom of the physics education research community, in which teachers develop content knowledge and then practice using that knowledge to listen and respond to student thinking.