Student engagement in learning science is both a desirable goal and a long-standing teacher challenge. Moving beyond engagement understood as transient topic interest, we argue that cognitive engagement entails sustained interaction in the processes of how knowledge claims are generated, judged, and shared in this subject. In this paper, we particularly focus on the initial claim-building aspect of this reasoning as a crucial phase in student engagement. In reviewing the literature on student reasoning and argumentation, we note that the well-established frameworks for claim-judging are not matched by accounts of creative reasoning in claim-building. We develop an exploratory framework to characterise and enact this reasoning to enhance engagement. We then apply this framework to interpret two lessons by two science teachers where they aimed to develop students' reasoning capabilities to support learning.