We present a class discussion that took place in the second author's high school biology class. Working from video data that we transcribed, studied, and analyzed closely, we recount how the question "Is air matter?" posed at the beginning of a unit on photosynthesis led to student-driven inquiry and learning. This case study illustrates what we argue is important in effective science teaching and learning: attending and responding to the substance of student thinking. We use it to articulate two reasons for attentive and responsive teaching: to help students understand science concepts, and to help students learn how to learn.