It is true that a piece of paper has two sides. Strictly speaking, the paper is a cuboid with six sides. However, without calculating the thickness, there are undoubtedly only two sides. Now this can be well reflected in the workbook and various applications.
Generally speaking, everything has two sides. We can regard the good side as the positive side and the bad side as the negative side. The same is true for paper. For example, our most common paper is actually a cuboid in a strict sense. When the thickness is not calculated, it has two sides both visually and spatially.
Therefore, it can also be said that a piece of paper has two sides, that is, the front and side in the general sense. This is also used in the current homework and draft. After one side is used up, you can continue to extract and transcribe on the other side, which is more environmentally friendly.
However, we can often turn the paper with two sides into one side in one way, and rotate one end of the paper 180 degrees to connect with the other end to form a mobius ring. No matter how we rotate, there is only one side, so as to make the other side small, but normally, there are undoubtedly two sides.