Just as a line is made of an infinite number of points, a plane is made of an infinite number of lines that are right next to each other. [br][br]A plane is flat, and it goes on infinitely in all directions. A sheet of paper represents a small part of one plane. But actually a sheet of paper is much thicker than a plane, because a plane has no thickness. It is only as thick as a point, which takes up no space at all. So a plane is like an imaginary sheet of paper, infinitely wide and long, but with no thickness.[br][br]When we talk about a triangle or a square, these shapes are like pieces cut out of a plane, as if you had cut them out of a piece of paper.  But is there another way to create these polygons or other shapes like circles?[br][br]Some geometers are very interested what happens when a plane intersects or cuts a 3-Dimensional shape.[br][br]Examine the [i]GeoGebra[/i] workspace.[br][br]The blue rectangle represents, like a piece of paper, a small part of a plane cutting through a cone.  [br][br]The red shape represents the shape that would be formed if the plane actually cut the cone.  [br][br]The green points are  drag points that can be used to reorient the intersecting plane.[br][br]Use the various sliders to experiment with different “slices” of the cube. [br][br]Observe what shapes are created when a plane slices through a cone.[br]What shapes did you observe?