How do military training instructors hold back from laughing?

Military training instructors, especially in the process of walking in the queue, sometimes refrain from laughing, because if they don't look, they won't see it at all. People walking in the queue are really funny, especially when there is no such rule and no such sense of unity at first. They are just closed, holding their mouths and almost holding their eyes. Of course, when we are not in military training, or when we have a rest and chat, the instructors will also talk to us about some topics in life. Sometimes when we talk that the seeds of our queue are instructors, we will laugh unconsciously, because we are not in the formal training process, so there is no need to hold back this emotion.