One day last week I was walking down the hall toward my office, past some offices my colleagues occupy. There were two students standing in front of the office of one of my colleagues, looking at the door carefully, examining it closely, testing the doorknob as quietly as possible, stuff like that. They saw me coming, and one of them said, very quietly, "do you know if he's in there?"
I said, "No. I don't know. Did you try knocking?" They hadn't.
The next day I was eating lunch in my office with the door closed. A student had made arrangements with me to take an exam that afternoon, but I had 15 minutes left before he was due. When he knocked on the door (very quietly) 15 minutes early, I had a mouth full of food, so I was going to wait until I could swallow it before I said, "come in." But I didn't have a chance, because he just opened the door and let himself in. I had to tell him that when you knock on someone's door and they don't say "come in" that means that they haven't given you permission to come in, and you should stay out.
Something similar happened just now. Door closed; student opens it; no knock at all. The guy didn't even knock. He just opened the door to my office. Is it normal for people not to know what the deal with doors is?