Hello, Younghan! I'm back after a long time to take a class while doing my first team project in my life. It was a great time to review the vague contents I knew in object-oriented programming! In particular, the part where you explained the memory structure when creating an instance in polymorphic reference + the priority of overriding was a decisive point that made me perfectly understand what I vaguely felt would happen. I think the part where you refactor the code in the problem will be really helpful to those who are studying to become developers, including me. Also, although this is not a review of this class, I realized how important it is to think about and implement problems in real life rather than just cloning code while doing this project! I watched the class with my eyes, but also typed it all along, changed it this way and that, and wrote it on my blog, but When I tried to develop the API that I actually had to implement, I couldn't remember everything I learned, and I had to refactor the code I made several times. While developing, I encountered a problem and read it again several times, thinking, "Ah, I think I've seen this somewhere," and I think I learned it more clearly and thought more about it! I am grateful that you always provide good, easy-to-understand classes! I'll see you in the next review!