PLEASE MOVE your mouse around square block.
Two perpendicular slots permit two prismatic sliders with a connecting link to oscillate between vertical and horizontal positions though a turning a crank (not shown in the animation, instead mouse movement rotates the connecting link). It is well known that an extension of the connecting rod will trace an exact ellipse. You can change the location of the red point ( on the connecting link) drawing the ellipse by using the slider. In which point of the red dot you can draw perfect circle?