Background and context: We address the question of what computer science students take the discipline to be. How students conceive the discipline can influence whether a student pursues computer science, what particular area within computer science they focus on and whether they persist in the discipline. In this paper, we examine the epistemic practices -- how knowledge is created and warranted -- of students and teacher as a way to understand what computer science is, as practiced by the participants. Objective: The goal of this study is to provide an account for how the students and teacher make and legitimize knowledge in the classroom and computer lab. Method: In this case study, we use a combination of ethnographic and multimodal methods. The data collected include audio and video recordings and researcher field notes of classroom and lab sessions. The analysis of the data considers speech, marks on the blackboard and embodied actions including gesture and gaze orientation. Findings: In both of these learning environments, CS knowledge in the form of general principles and programming practice are discussed by students and the instructor. However, the way in which such knowledge is warranted in these two settings is different. In the classroom, the instructor is the authority that warrants knowledge claims, whereas in the computer lab, the computer plays a key authority role to determine "what works". Implications: This study provides insight into how instructors can design learning environments based on ways they intend knowledge to be created and warranted.