Language is the way students and teachers communicate in the science classroom, but language of science is not always the language children and adults use in their everyday life. As Michaels, Shouse, and Schweingruber put it, "In science, words are often given specific meanings that may be different from or more precise than their everyday meanings. It is important for educators to be clear about specific scientific usage to avoid confusion" (2008, p. 4). For example, consider the word theory. How many times have you heard someone say (or have you caught yourself saying), "That's just a theory" or "I have a theory about that"? This colloquial use of the word theory usually implies a hunch, guess, belief, and even an unsupported prediction. The formative assessment probe "Is It a Theory?" is used to elicit ideas about the nature of science. This probe is designed to find out if students (and teachers) distinguish scientific theories from the common use of the word "theory" and if they understand how theories differ from laws. This formative assessment probe was developed for use in middle and high school science, the grade levels when students become familiar with the historical and current developments that led to scientific theories and begin to distinguish between facts, hypotheses, theories, and laws. The formative assessment probe and students' discussions as they use the word "theory" can both reveal common misunderstandings students have about the word "theory" when it is used in science. By being aware of students' misunderstandings, making instructional decisions to use the word "theory" to mean explanations supported with evidence that help us understand why something happens, and constantly monitoring how students use their evolving scientific language, elementary teachers can have a significant role in shaping students' understanding of the nature of science and the specific language that it includes.