Why does a triangle have stability and a bench has four corners?

the stability of a triangle refers to the stress stability of a triangle, provided that the triangle has three real sides. There are only three points at the foot of the bench, so the triangle stability is not satisfied. Theoretically speaking, the more legs there are (of course, the legs are almost the same length), the more stable the bench will be in the butt plane < P > This is a mechanical problem, not a mathematical problem.