What do people mean when they say they "know" something in science? It usually means they did an investigation and expended considerable intellectual effort to build a useful explanatory model. It means they are confident about an explanation, believe others should trust what they say, and believe that their claim is testable. It means they can expect to be challenged and called to defend their position, and that their interpretation could eventually be proven "wrong" someday. In addition to these practical implications, when an individual says "I know," it reveals something about his or her worldview and experiences, maybe even individual motives and goals. In other words, knowledge is inherently personal. Learning is a process that can lead the development of a child, and learning science can influence whether a child wants to be someone who engages in exploration, explanation, and argumentation. There are a growing number of resources to help elementary students structure scientific arguments, write arguments in science, and evaluate their own arguments and others' arguments. As prerequisites to learning the form of a high-quality scientific argument, it is important to address not only the link between claims and evidence, but also the role of evidence within the overall process of doing science, including processes of thinking, reasoning, and knowing. This broader focus allows for a deeper grasp of evidence such as making sense of the relationship between evidence and a claim. In this article, the authors present a summary of one of the five units on scientific evidence that they developed, "What Is Knowledge and Evidence?" "What Is Knowledge and Evidence?" is a unit designed to engage students in examining how scientific knowledge is developed and shaped over time. Based on the authors' findings from tests in classrooms, they found this unit (which can be conducted in one week for approximately 30-45 minutes a day) provides elementary school students with an introduction to the purpose that evidence serves in science. This type of introduction to knowledge is an important prerequisite to learning common scientific forms of reasoning. (Contains 5 figures.)