In this paper, I hypothesize that students develop their understanding of science by indexing scientific terms or principles with objects in the environment or events occurring in current or previously experienced situations. From this perspective, knowing a scientific concept amounts to knowing the utility of the concept and the situations in which it can be used. This hypothesis is supported by evidence from an empirical study of a sequence of nine lessons on the topic of “States of Matter” in a Grade 7 Australian classroom. In particular, I will illustrate the indexicality of scientific understanding through analysis of a classroom activity that required the students to classify “slime”. Rather than employing those official properties of solids, liquids, and gases as provided by the teacher, the focus students based their classification decisions on the similarities and differences between slime and those familiar objects that they considered to be legitimate exemplars of solids or liquids. This study highlights the impact of the particular employment of instructional resources and the role of student experience in sense making of science in classroom settings. The contribution of student experience is contrasted with the logical structure of scientific categories in the organization of an individual’s knowledge.