Other curves: conics
The ellipse,
This display illustrates a property of all ellipses. Any ellipse has two points called the foci - points F1 and F2 in the diagram. Any point on the ellipse has the property that the sum of its distances from F1 and F2 is always the same, and equal to the length AB. Move point F1 or F2 to see this. You can change the distance AB too. (Note that AB has to be greater than the distance between the foci; otherwise you get another conic curve altogether.)

Conic curves The ellipse is an example of a conic curve. This means that you can make a slice (shown in green) through a cone that creates a elliptic cross-section (shown in red).