So, in plane we have three things one is an adjuster of the boundary, one is the interior and one is the boundary itself. So cussed cops now, can you go from one interior point to an exterior point without crossing the boundary it is not possible. So, from one interior point, if I have to go to exterior point; that means, we have to cross the boundary at some point at least at one point and in. So, this is what this is a characteristic of a simple closed polygon; one is the boundary, one point on other is you can always follow. You have to follow particular path; the second thing that from one point to another point. If want to go from one interior point to exterior, you have to cross the boundary at some point and the other things the boundary divides the points of the plane into 2 different regions; one is the exterior region, one is the interior region and the interior such interior point such that any 2 points can be join, can be joined by path which is again inside the boundary where as 2 exterior points. If you take any 2 points; if you join them, it has to be outside.