This becomes even more apparent when you turn the circumference of the fixed circle into a straight line. (As the video suggested but didn't show in the same wa

This becomes even more apparent when you turn the circumference of the fixed circle into a straight line. (As the video suggested but didn't show in the same way.) The center of the rolling circle will trace a line parallel and equal length to the fixed line, whether above or below it. If you reform the circle the outer line will be too short to form a complete circle, and the inner line will overlap itself.