We create a parabola, using the definition: A parabola is a curve formed by the set of points in a plane that are all equally distant from both a given line (called the directrix) and a given point (called the focus) that is not on the line. Line AB is the directrix and Point C is the focus. By construction, point E is equidistant from point C and point D. Therefore the construction fits the definition of a parabola, which is why a curve is formed.