This paper investigates how features of teaching practice may foster appropriation: learning that involves both deep conceptual understanding and transforming scientific discourse in a way that is significant for oneself. Data were drawn from classroom discussions during a unit on thermodynamics that took place in a high school in Italy. We analyze a specific metaphor "tightening the reins and letting them loosen" used by the teacher to characterize her practice. By contrasting episodes of classroom discussion in terms of talk moves and participant frameworks, we discovered that behind the metaphor was a complex epistemological scaffolding that the teacher enacted across her lessons. This scaffolding was articulated across four moments of discussion, each with a different function and purpose: Sharing the construction of a collective disciplinary narrative, Elaborating the narrative in epistemological terms by articulating criteria for comparing different approaches to the disciplinary content, Analyzing the comparison criteria to test their robustness and consistency, and Situating oneself with respect to different approaches to the content informed by the refined criteria (SEAS). We argue that these functions provided the scaffolding for physics to be a context for both disciplinary learning and students' search for personal relevance and meaning.