Read about some interesting postings on Bezier curve as used in Flash drawing API today.
http://www.timotheegroleau.com/Flash/articles/cubic_bezier_in_flash.htm
http://www.actionscript.org/tutorials/advanced/Understanding_curves_and_control_point_placement/index.shtml

To sum it up basically Flash uses a quadratic Bezier curve, versus the more traditional cubic Bezier curve used in commercial vector graphic software. With quadratic curve (to refresh your memory of high school math, quadratic equation is an algebraic equation with the power term raised to power of 2, as in a*x^2 + b*x + c). So visually it means in Flash drawing API, you have two anchor points and one control point, giving you a quad curve. In order words, you can never get a closed circle or loop using just one curve.