The national efforts underway to include engineering in K-12 science education present a variety of new challenges, including how to prepare teachers to teach a new discipline. In this paper, we focus on elementary teachers and how they enter into "responsive teaching," in which they closely attend and meaningfully respond to students' thinking. This pedagogical approach is particularly critical for teaching engineering design in ways that support students in developing original solutions to complex, dynamic problems. Drawing on 3 years of professional development that helped teachers incorporate engineering in literacy contexts, we investigated the knowledge and abilities teachers displayed for responsive teaching and how features of instructional design and classroom dynamics impacted their teaching. Throughout the project, we observed and videotaped professional development workshops and teachers' classrooms, and interviewed teachers about their classroom experiences. We then conducted a thematic analysis across these data sources, presenting common themes as conjectures for how teachers enter into responsive teaching in engineering. We discuss how these conjectures can inform teacher research and the design of teacher preparation programs in engineering.