The six sentence argument (6SA) is a critical thinking exercise that can be easily integrated into existing curricula. We present students with a case and ask them to argue for their preferred course of action using exactly six sentences. Through a guided peer review component, students engage critically with other students’ writings and receive detailed feedback on their arguments. This exercise helps students learn to craft brief, convincing arguments and to reflect on their reasoning in a format that can also be applied in many practical situations. The 6SA works well for undergraduate classes, and can also be used to prepare a case discussion at graduate level. Due to the peer review element, the exercise can be scaled up to accommodate large classes with little additional work for the teacher.