Thank-you notes make people happy. For as much joy as they give me, I don’t send them enough. In fact, I think writing thank-you notes is a dying art. They’re overlooked forms of positive closure. Gratitude on its own is powerful, and when it’s exchanged, it feels amazing. After I thought about what notes of gratitude could accomplish, I started emailing thank-you notes to my students, waiting until well after the semester for the most impact.