Why is a donkey's mill round?

Because the edges of the tool used to pull the mill are the same distance from the center, so it's smoother that way. When a line segment is rotated around one of its endpoints in a plane, the path of its other endpoint is called a circle. The center of the mill is the previous endpoint, the center of the circle; the distance between the donkey and the center of the mill is fixed, representing that one line segment, the donkey is the other endpoint; the donkey pulls the mill on a course equivalent to the trajectory of the other endpoint as it rotates around it in the plane for one week, which is a circle.

Circle:

A circle is a geometric figure. The diameter of a circle within the same circle and the length of the radius are always the same, and a circle has an infinite number of radii and an infinite number of diameters. A circle is an axisymmetric, centrosymmetric figure. The axis of symmetry is the diameter of the line. At the same time, the circle is a "positive infinite polygon", and "infinite" is just a concept. A circle can be viewed as a square polygon consisting of an infinite number of infinitely small points, and when the polygon has more sides, its shape, perimeter, and area are all closer to that of a circle. Therefore, there is no real circle in the world; a circle is really just a conceptual figure.