Dynamic parallelogram

A parallelogram is a quadrilateral with two pairs of parallel sides. (Note that this includes rectangles, among other types of figures.) Move B to and C to change the side lengths and directions. To gradually change a point, select the point and hold down an arrow key.
Try moving B onto the circle. What kind of polygon do you get? Try making a rectangle. Try making a square. Part of the textbook Measuring the World: Mathematics for Elementary and Middle School Teachers by Susan Addington and David Dennis. Copyright 2009.