History why do people shake their head to mean no

In my opinion it's just natural. So many times I have gave my children the answer to their questions just through a simple look. Especially in church or other public places. Body posture, gestures and facial expressions is a universal way of communicating, actually another language. At times I have caught those around me in a lie. The mouth says one thing but body posture and the eyes are saying something totally different. Study the people around you. Ask questions or just talk (don't tell them what you are up to) and not only listen to what they say but watch the body movements, hand gestures and their eyes carefully. You will begin to see that a good 8 times out of 10 you would get their meaning without ever hearing a word from their mouth. Hope this helps and have fun with it. Happy holidays!