Suppose you are given two distinct points in the plane, [i]A[/i] and [i]F[/i]. Draw a parabola that passes through [i]A[/i] and has [i]F[/i] as its focal point. Suppose [i]B[/i] is the vertex of this parabola. There are infinitely many such parabolas. The set of all possible locations for [i]B[/i] is a cardioid.[br][br]In the applet below, [i]A[/i] and [i]F[/i] are fixed, and you can obtain different parabolas by moving the vertex [i]B[/i].