Research carried out through the last 20 years gave us undeniable evidence that to learn anything we need to be active participants, not passive observers. One of the important aspects of learning physics is constructing explanations of physical phenomena. To support and guide students toward constructing their explanations, teachers need to be attentive and responsive to students' explanations. To learn how physics teachers interpret and respond to students' explanations we investigated pre- and in-service physics teachers' responses to students' written explanations of their answers to a complex physics problem. The survey administered to the participants included the problem statement and four authentic student explanations. The participants were asked to identify each student's strengths and weaknesses and to provide a response to that student. We found that while the participants were successful in identifying productive and problematic aspects of student reasoning, they rarely built on student reasoning when responding to the students, mostly focusing on addressing problematic aspects. The paper discusses why this finding is important for physics teacher preparation programs and professional development programs.