Why can't the paper be leveled after it becomes uneven?

As we all know, the raw material of paper is pulp, which is a cellulose synthetic substance extracted from wood by mechanical and chemical treatment methods. So before answering this question, we need to briefly introduce how pulp is made into paper. . "First, the pulp is soaked in water and beaten to finely separate cellulose, and then stirred to disperse cellulose as evenly as possible. Finally, use a machine called a paper machine to copy it into layers and dry it, and the paper is made. " Here, let's look through the paper, and you will see that some places are white and some places are black. This is because cellulose is unevenly dispersed. There is less cellulose in white soil and more cellulose in black soil. It can also be understood that the surface of the base paper will be uneven without any pressure, and it should be leveled by mechanical means and external force. "In the process of producing paper, it should be stretched vertically to make it dry in the stretched state. The last process should be pressed between the rollers to make the surface smooth. The reason why wet paper can't return to its original flat appearance is that water penetrates into the flattened cellulose and expands again. Cellulose is a porous substance with strong water absorption. " But if paper really absorbs water so easily, then if we write with a pen, won't it melt and become unrecognizable as soon as we write? In fact, this will not happen. What kind of treatment has been given to the produced paper? Mr. Kimura told us: "In addition to special purpose paper such as filter paper for separating substances in liquid and absorbent paper for absorbing water, the produced paper must be treated with waterproof technology. Before entering the paper machine processing process, a certain amount of water-resistant drugs called rosin extracted from rosin should be added according to the different uses of paper products. " The paper used to make notebooks and so on, as well as the paper used to print textbooks and books, must be treated with waterproof technology, so that water will not immediately penetrate into it when printing or writing.