It's rude to call someone dumb.
Joe is sitting in a large auditorium with his classmates for their history lecture.
Joe wants to chat with the student next to him.
Joe inquires how the person next to him is liking the class they are taking.
Another student misses a question the professor asks and Joe comments about what a dummy they are to the person next to him.
Joe makes a new friend and study partner.
Joe is called out by those around him for being disrespectful and mean.