Formative assessment, and especially feedback, is considered essential to student learning. To provide effective feedback, however, teachers must act upon the information that students reveal during instruction. In this study, we apply a framework of formative assessment to explore how sensitive teachers are to students' thoughts and ideas when teaching for conceptual understanding. Six elementary school teachers were interviewed and videotaped as they implemented a curriculum that emphasized the teaching of key science concepts through different modes of learning (doing, reading, writing, and talking). We created four main categories for fostering conceptual understanding: identifying learning goals, eliciting student information, interpreting student information, and acting. Findings indicate that elementary school teachers with low levels of pedagogical content knowledge in science do not always know the key concepts of a scientific idea or how to teach them to increase student learning. Therefore, teachers' interpretation of students' responses and their subsequent actions are not likely to be aligned to the scientific idea the key concepts represent. We suggest that teachers need support to identify the key concepts within the discipline of science. Equally important is to realize that to make meaning, these concepts must be taught in a context and in relation to other words within the discipline.