Peer learning has become a central feature in many postsecondary classrooms. Many peer learning activities feature near-peer instructors who had previously completed the course and who later serve as peer leaders. There are relatively few examples of activities in which students in a course act as true peer instructors for their classmates. One such activity is described herein. This activity, "Teach What You Know Day" (TWYKD), was implemented in an advanced organic chemistry course. Students in this course had varied background preparation and previous knowledge. Students capitalized on this knowledge to present course topics to their peers. During this activity, the peer leaders gave a lecture and helped their classmates work through practice problems. Assessment of this activity was conducted via a student perception survey and by comparing exam scores on TWYKD and non-TWYKD-related questions. The survey results indicate that the students generally had a positive reaction to this assignment. The exam assessment shows that students were able to learn from their peers. This assignment provides many benefits to students enrolled in upper division chemistry courses. With some personalization of the lecture topics, TWYKD would be a valuable addition to other upper division chemistry courses.