The first answer is why crowding occurs. Some scholars once studied the skull and teeth of ancient humans and found that tens of thousands of years ago, humans had crowded teeth. Because with the continuous development of human civilization, human recipes slowly changed from raw food, coarse food to cooked food, fine food; food is more and more fine, chewing less and less, resulting in muscle slowly atrophy. The order of degeneration of human muscles, jawbones and teeth is: muscle atrophy, then to jawbone reduction, then to the number of teeth reduction. Nowadays, human chewing muscles have atrophied and jawbones have become smaller, but the number of teeth has hardly decreased; instead, because of the improved nutrition now, teeth have grown bigger. Therefore, the teeth are often crowded and have to be extracted in order to achieve an aesthetic harmony between teeth and bone. Why does protrusion occur? First of all, it is a matter of ethnicity. We belong to the Mongolian race, and the Mongolian race is characterized by a low and flat nose and protruding jaws. Secondly, it is the problem of tooth volume – bone volume incongruity, which is the same as the reason of crowding. Again, it is a bad habit. Some friends like to bite their lips, especially children, this is especially prominent. Biting the upper lip will cause the lower jaw teeth to protrude, and biting the lower lip will cause the upper jaw teeth to protrude. There is also the incongruity of the upper jaw jaw. Overdevelopment of the upper jaw or underdevelopment of the lower jaw will lead to the protrusion of the upper jaw teeth; the opposite will lead to the protrusion of the lower jaw.