We further conceptualize the work of responsive teaching by contrasting it with a type of teaching we call endorsing. We define responsive teaching as a type of teaching in which the teacher: (a) ensures students' ideas are the genesis of ways of reasoning that are eventually taken up by the classroom community; and (b) continually assesses and adjusts the focus of class discussions to meet the evolving intellectual needs of the students. In contrast, endorsing occurs when a teacher uses student thinking to advance the mathematical agenda by selectively developing ideas that align with their goals and vision of how the ideas should be developed in class. This type of teachings lacks the second key feature of responsive teaching. We argue that endorsing can be problematic for students' individual learning and discuss implications for teacher development.