Exterior Angles of Non-Convex Polygons
The sum of the exterior angle of any polygon is 360˚. But it can also be shown to be true of non-convex (concave) figures as well. If we make our lines for our exterior angles in the usual way, when we come to the inverted angle, the exterior angle will be on the inside of our figure, so this is bad. But, if we rotate in the opposite direction, we have a negative angle and the formula still works! Fun!