A "Framework for K-12 Science Education"'s disciplinary core idea PS1.A states that students should know by the end of grade 2 that different kinds of matter exist and many of them can be solid or liquid, depending on temperature (NRC 2012). By the end of grade 8, they describe solids, liquids, and gases by the arrangement and motion of their molecules. But what about "in between" ideas? In the elementary grades, students are typically taught to define solids and liquids using macroscopic properties. Solids generally keep their shape and have a definite volume. Liquids have a definite volume but can take the shape of their container. Sometimes this definition is expanded to include that liquids can be poured. This definition using shape and pouring can be problematic when tiny parts of solid materials, such as powders or granules, are involved. In addition, the everyday use of the word solid implies something that is hard and not soft or "airy." The assessment probe, "Is It a Solid?" (Figure 1; Keeley, Eberle, and Dorsey 2008) can be used to elicit elementary students' ideas about solids and the macroscopic properties they use to decide whether a material is a solid. It can reveal whether students have developed misinterpretations based on early definitions of solids and liquids and the familiar use of these words. Furthermore, when combined with the formative assessment strategy known as "claim cards," the probe also provides insight into how elementary students engage in the scientific practice of argumentation. This article provides an example of a teacher's implementation of claim cards during a science talk in a fifth-grade classroom. The example demonstrates how teachers can use formative assessment to uncover and--through carefully designed instruction--confront students' ideas about the macroscopic properties of solids before they move onto the microscopic properties in middle school. At the same time, it provides an informal assessment window into students' ability to state claims and engage in argumentation.