In mathematics, an ellipse is a trajectory on a plane where the sum of the distances to two fixed points is a constant. These two fixed points are called foci.

From this definition, you can draw an ellipse in this way: first prepare a line, tie each end of this line to a point (these two points are taken to be the two foci of the ellipse); take a pen, and tauten the line, at which point the two points and the pen form a triangle; then pull the line and start graphing, continually making the line taut, and finally complete the figure of an ellipse.