Plane Cross Sections of the Surface of a Cone
This is a simple Demonstration of the cross sections of the surface of a cone. It shows not only the nondegenerate conics, that is, the ellipse, the hyperbola, and the parabola, but also the degenerate conics (which are a single point), a straight line, and a pair of intersecting lines.
See the Demonstrations in Related Links to easily find some interesting preset positions of the plane.
For more theory see .
 Wikipedia. "Conic Section." (Dec 31, 2013) en.wikipedia.org/wiki/Conic_section.