ABCD is a quadrilateral. Click and drag any of the blue points to change the shape of the quadrilateral.

- What observation can you make regarding the angle sum of a quadrilateral?
- Move the slider to the right. What relationship exists between the angle sum of a triangle and the angle sum of a quadrilateral?
- Move the point B to a position so that <ABC is a reflex angle. What is the special name given to such a polygon?