Given two points, and (the foci), an ellipse is the locus of points such that the sum of the distances from to and to is a constant. A hyperbola is the locus of points such that the absolute value of the difference between the distances from to and to is a constant. An oval of Cassini is the locus of points such that the product of the distances from to and to is a constant ( here). A parabola is the locus of points such that the distance from to a point (the focus) is equal to the distance from to a line (the directrix). This Demonstration illustrates those definitions by letting you move a point along the figure and watch the relevant distances change.