Like a multitude of other things, the signs which we give by the movements of our heads to indicate "yes" and "no" were copied from animal life.

When the mother animal brought her young a choice morsel of food she would hold it up temptingly before its mouth and the quick forward movement of the head, with mouth open, showed the young animal's desire and acceptance of the offer. Even today when we make a forward movement of our heads to indicate "yes" it is observed that the lips are usually quite unconsciously opened a little.

In much the same manner, when the young had been well fed and were no longer hungry, a tightly closed mouth and a snaking of the head from side to side were resorted to, to keep the mother from putting the food into their mouths. Our natural impulse now is to slightly clinch our teeth when we shake our heads to mean "no/'