In responsive teaching, instructors seek to understand and pursue the substance of students' thinking by foregrounding connections between students' ideas and disciplinary understandings. Education research literature suggests that responsive teaching benefits student learning, yet is also difficult to implement in fast-paced science courses, including university physics courses. This may be one reason there are few examples of responsive teaching at the university level. We share an example of responsive teaching from a small group conversation about heat and temperature in an introductory, calculus-based university physics course.  In this example, an instructor proposes a thought experiment that takes up and advances students' thinking about heat and temperature. This example illustrates that responsive teaching is possible in university-level courses, and suggests that one way university science instructors can bridge students' thinking and sophisticated content learning goals is by posing carefully-selected "thought experiments" that target the connections between students' thinking and scientific models.