There are two electric lights hanging on two wires and on the ceiling. Are these two wires parallel? Nature is parallel, because they are perpendicular to the ground! But think about it. Why do electric lights sag? This is because of the influence of gravity. In this case, if two wires are extended, they will intersect at the center of the earth. That is to say, the straight lines of two wires hanging from the ceiling are not parallel, but intersect.
We regard sunlight as parallel, which seems to be no problem. However, where does the sunshine come from? It comes from the center of the sun. In other words, sunlight comes from a little light.
Therefore, from the perspective of human life, we regard the sunlight as parallel, and even the two wires hanging from the ceiling are regarded as parallel, which is also good. But objectively speaking, it is more reasonable to regard light or even drooping wires as intersecting straight lines. It can be seen that the research problem depends on the scope. A conclusion is correct in a small range, but it is not necessarily correct in a large range. At present, geometry in primary and secondary schools mainly studies the graphic properties in a small range, that is, Euclidean geometry. If we study in the macro world or micro world, there will be non-Euclidean geometry, which is the content of Robachov geometry and Riemann geometry.