If your teacher's pants are not zipped, how can you tell him so as not to embarrass the teacher?

Hehe, I have seen such a joke before. If I were you, I wouldn't say it under normal circumstances, in case the teacher is too embarrassed, but not saying it will cause more embarrassment. But my inner thought is to say to the teacher very seriously: "teacher, your door is open." You will know when you go to the toilet. " It's just my inner thoughts. In reality, I will never speak.