Previous research on peer tutoring has found that students sometimes benefit academically from tutoring other students. In this study we combined quantitative and qualitative analyses to explore how untrained peer tutors learned via explaining and responding to tutee questions in a non-reciprocal tutoring setting. In support of our hypotheses, we found that tutors learned most effectively when their instructional activities incorporated "reflective knowledge-building" in which they monitored their own understanding, generated inferences to repair misunderstandings, and elaborated upon the source materials. However, tutors seemed to adopt a "knowledge-telling bias" in which they primarily summarized the source materials with little elaboration. Tutors' reflective knowledge-building activities, when they occurred, were more frequently elicited by interactions with their tutee. In particular, when tutees asked questions that contained an inference or required an inferential answer, tutors' responses were more likely to be elaborative and metacognitive. Directions for future research are also discussed.