Wheels are gradually discovered by humans in the labor practice of carrying things. In the primitive society, human ancestors made a living by collecting and hunting. At the beginning, the food was limited, and it was easy to bring it back with you. So, some people came up with the idea, picked a few broken thick branches from the ground, connected the branches with vines, and then placed the prey on it, grabbing the two long branches with both hands and dragging it away. The shoulders are much lighter. I also thought of using a wooden stick, one end on the shoulder, the other end on the ground, hanging the heavy object in the middle of the wooden stick, which is also more labor-saving than hand. The man put the two sticks side by side, with a piece of cloth in the middle, holding both ends of the stick and the other end dragging on the ground. All of the above are the kind of "small squat" that people originally invented, which is characterized by sliding on the ground by means of a slide bar.