In this animation you can see two circles (the blue one inside the red one) which lie on a plane which is orthogonal to the green plane, hence an inhabitant of the two dimensional green world can see only the trace of the two circles. Since the green plane is moving, the trace is given by moving points. In this first example, we have added, for clarity, a second picture with the configuration of the circles as can be seen from a three dimensional point of view.  back