Teaching that is responsive to students' ideas can create opportunities for rigorous sense-making talk by young learners. Yet we have few accounts of how thoughtful attempts at responsive teaching unfold across units of instruction in elementary science classrooms and have only begun to understand how responsiveness encourages rigor in conversations. In this study, the first author taught an electric circuits unit to four upper elementary science classes, exercising a responsive teaching stance. We found that rigorous episodes of whole-class talk were associated with the teacher's use of open-ended questions, follow-up prompts, references to activity or representations, prediscussion tasks, and asking students to comment on their peers' ideas. Overall, higher rigor talk co-occurred with these conditions when used in combination. Despite being responsive to students' emerging ideas, all four classes addressed the science ideas for the unit--an outcome we attribute to the use of an anchoring phenomenon and the teacher's awareness of the concepts required to construct evidence-based explanations for it. Finally, concerted attempts to teach in responsive ways--while also attending to rigor--surfaced pedagogical tensions that problematize efforts to create such discourse-rich environments and inform how this type of instruction might be enacted by others.