This quasi-experimental and pre/posttest study was designed to examine whether fourth-grade students who engaged in collaboratively writing letters to 11th-grade students performed better on tests of conceptual understanding of a unit on force and motion than students who did not. The participants included 835 fourth-grade students and 416 11th-grade students from four elementary schools and one high school in the United States. Students in treatment groups were asked to write three letters for exchange at the beginning/during/completion of an 8-week teaching unit about force and motion. The structure of writing was based on three components of arguments: question, claim, and evidence. A comparison of the pre-and posttest results indicated that students who engaged in collaborative letter-writing tasks performed better than students who did not. The pre-and posttest results also showed that female, special, low socioeconomic status, and gifted students benefited the most from the collaborative letter-writing tasks. Through the letter-writing exchange activities, the fourth-grade students were supported and encouraged to learn difficult concepts by the 11th-grade students. The more the 11th graders asked the fourth graders for explanation and clarification of the concepts in the letters, the more the fourth-grade students learned. Two factors related to argumentative writing were identified as predictors of success on students' achievement tests: overall cohesiveness, with an emphasis on embedding multiple modal representations in the text and the strength of the relationship between claims and evidence. This study suggests that there is a pedagogical need to build support structures to help students understand the role of audience, the function of multiple modal representations, and the quality of evidence for supporting knowledge claims in argumentative writing, as this could lead to a better understanding of scientific concepts.