Over the past two decades education researchers have demonstrated that various types of knowledge, including pedagogical content knowledge, influence teachers' instructional practices and their students' learning opportunities. Findings suggest that by engaging in the work of teaching, teachers acquire knowledge of how students think, but the education research community has not yet captured this learning as it occurs. During an investigation of whether novice physics instructors can develop such knowledge via the activities of attending to student work, we captured instances of knowledge development and have identified several mechanisms that supported instructors in building this knowledge. We analyzed data from interviews with physics graduate teaching assistants as they examined and discussed students' written work. During those discussions, some instructors appeared to develop new knowledge–either about students' thinking or about the physics content--and others did not. We compare and contrast three cases representing a range of outcomes and identify factors that influenced the development of new knowledge.