The six-sentence argument (6SA) is an exercise to train critical thinking skills. Faced with a decision situation, students argue for their preferred course of action using a logical structure of exactly six sentences. Through a guided peer review, students engage critically with other students' arguments and receive detailed feedback on their own arguments. This exercise helps students craft convincing arguments and reflect on their reasoning in a format that can be applied in real-world situations. A key strength of the six-sentence argument exercise is that it can be administered online and is scalable for large courses with little additional workload for the instructor.