A window frame in the shape of a trapezoid rotates on a pedestal. By watching it from a distance, it appears to swing back and forth in alternating rotations of 180Ü, rather than continually spinning a full 360Ü. This occurs because we are, in our civilization, surrounded by rectangular structures; hence when we are presented with a rotating trapezoidal object which has all the usual features of a common rectangular object, we are strongly inclined to see the smaller end as being further away. It is easier to believe that the rotation is reversing than to believe that the smaller end is actually getting closer.