What is a three-dimensional figure?
A three-dimensional figure, sometimes called a solid figure, is a set of plane regions and surface regions, all lying in three-dimensional space. These surface regions are called the faces of the figure. Each of them is two-dimensional. The arcs of curves that are the edges of the faces of the figure are called the edges of the figure. They are one-dimensional. The endpoints of the edges are called its vertices. They are zero-dimensional. The most common three-dimensional figures have only a few faces, the surfaces are very simple, and there are no “loose ends” – that is, every vertex is the end of at least two edges, and at least two faces meet at every edge. If all the faces are plane regions, every edge is the edge of two faces, every vertex is the vertex of at least three faces, and no two faces cross each other, the figure is called a polyhedron.2 Polyhedra are classified according to the number of faces. Here are some names of polyhedra. Polyhedra often divide space into two piec
