To draw a perfect ellipse.
To draw a perfect ellipse, stick two pins in a sheet of paper. Then make a string into a loop and place the loop over the pins. Stretch the string taut with the point of your pencil and then move the pencil around the pins. The pencil will trace a perfect ellipse. This shows the geometrical property of the ellipse which is made by lines drawn from the two foci to any point on the perimeter of the ellipse always have a constant sum. The two pins are at the foci and the string segments AC and BC are the two lines to a common point on the curve. Since AB stays the same length, the sum of AC and BC will remain constant while the pencil traces the curve. If the pins are moved closer together keeping the same size loop, the pencil traces wider ellipses.