Primary sources, as defined by the Library of Congress, are the "raw materials of history--original documents and objects which were created at the time under study" (Library of Congress, 2014, p. 1). Primary sources differ from secondary sources, which interpret events without the benefit of firsthand experience. While often employed in English, history, and social science courses, primary sources also can be used in science classes to help students acquire knowledge and become more engaged in their coursework, while supporting "Next Generation Science Standards" (NGSS Lead States 2013), "Common Core State Standards" (NGAC and CCSSO 2010), and 21st-century skills (Partnership for 21st Century Skills 2014). This article describes a framework called SOURCES, which is an acronym for a series of steps teachers can follow to integrate primary sources in the science classroom. The unit the authors offer as an example has students address the question of whether Earthlings are alone in the universe. While this activity, which can be completed in five days, centers on Earth and space science, the method can easily be adapted for use in any science discipline.