Background/Context: There are few examples from classrooms or the literature that provide a clear vision of teaching that simultaneously promotes rigorous disciplinary activity and is responsive to all students. Maintaining rigorous and equitable classroom discourse is a worthy goal, yet there is no clear consensus of how this actually works in a classroom. Focus of Study: What does highly rigorous and responsive talk sound like and how is this dialogue embedded in the social practices and activities of classrooms? Our aim was to examine student and teacher interactions in classroom episodes (warm-ups, small-group conversations, whole-group conversation, etc.) and contribute to a growing body of research that specifies equity in classroom practice. Research Design: This mixed-method study examines differences in discourse within and across classroom episodes (warm-ups, small-group conversations, whole-group conversation, etc.) that elevated, or failed to elevate, students' explanatory rigor in equitable ways. Data include 222 secondary science lessons (1,174 episodes) from 37 novice teachers. Lessons were videotaped and analyzed for the depth of students' explanatory talk and the quality of responsive dialogue. Findings: The findings support three statistical claims. First, high levels of rigor cannot be attained in classrooms where teachers are unresponsive to students' ideas or puzzlements. Second, the architecture of a lesson matters. Teachers and students engaging in highly rigorous and responsive lessons turned potentially trivial episodes (such as warm-ups) of science activity into robust learning experiences, connected to other episodes in the same lesson. Third, episodes featuring one or more forms of responsive talk elevated rigor. There were three forms of responsive talk observed in classrooms: building on students' science ideas, attending to students' participation in the learning community, and folding in students' lived experiences. Small but strategic moves within these forms were consequential for supporting rigor. Conclusions/Recommendations: This paper challenges the notion that rigor and responsiveness are attributes of curricula or individual teachers. Rigorous curriculum is necessary but not sufficient for ambitious and equitable science learning experiences; the interactions within the classroom are essential for sustaining the highest quality of scientific practice and sense-making. The data supported the development of a framework that articulates incremental differences in supporting students' explanatory rigor and three dimensions of responsiveness. We describe implications for using this framework in the design of teacher programs and professional development models.