Being responsive to the substance of students' disciplinary thinking is central to current visions of reformed science teaching. Studies of classroom practice, however, show that this kind of teaching is relatively rare. In this paper, we use data from one cohort of university physics Learning Assistants (LAs) to illustrate what it looks like for teachers to sense-make about being responsive to student thinking. We draw on the theoretical lens of sense-making--including Spillane et al.'s (2002) notion that teachers need opportunities to sense-make not only about the substance of new instructional approaches but also their own relationship to these approaches--to unpack the central questions that LAs asked about being responsive. We give evidence from course assignments that these LAs' extended sense-making was productive both (1) for their attention to the substance of student thinking and (2) to their identification with responsiveness as a central facet of good instruction, Our case study highlights the tensions that teachers may experience as they enter this instructional space and what "resolution" of such tensions can look like.