Writing for an audience different from teachers motivates students to translate their existing knowledge into audience-appropriate language, in which students explain, elaborate, and integrate their understanding of science concepts using more than just the technical language of the subject. Several studies also have found that students can achieve deeper conceptual understandings when they engage in argumentative writing. This article introduces a writing-to-learn activity called "Writing an argument to a real audience" in which students craft an evidence-based argument consisting of claims and evidence to persuade the audience regarding their ideas. Student participation in this activity could lead to a better understanding of scientific concepts.