This is to illustrate that a plane section of a cone is an ellipse because the sum of the distances to the two foci is constant. [br]Consider the two spheres that are tangent to the cone and to the plane. They touch the plane in two points thats appear to be the foci of the ellipse. [br]The reason why the coloured (green and cyan) segment lines are congruent is left to the reader. :-)[br]The diagram shows why their sum is constant.