This article articulates how a "grasp of practice" serves as a reasoning resource for inquiry and citizenship abilities associated with nature of science (NOS) understanding. Theoretically, this resource is elaborated through an overlapping concern with "practice" in two literatures, science studies and psychology of learning, bringing attention to two key roles in scientific practice, Critiquers and Constructors of claims. Empirically, this resource is made plausible by the results of an expert-novice study and a classroom study. In the expert-novice study, reactions of scientists and laypeople to science-related claims in the popular media were contrasted, underlining the appropriate ways scientists tend to Critique such claims. In the classroom study, sixth-grade students engaged in a 2-week ramp experiment, experiencing first hand the roles of Critiquers and Constructors of claims, and were subsequently assessed with a novel experimental task. Performances suggest that students had attained a grasp of practice, going well beyond mere execution of methods or procedures. These results challenge a common assumption that declarative knowledge best characterizes learning targets for supporting inquiry and NOS understanding.