How to Recognize a Polyhedron
A polyhedron is the three-dimensional equivalent of a polygon, which is a shape that has only straight sides. Similarly, a polyhedron is a solid that has only straight edges and flat faces (that is, faces that are polygons).
The most common polyhedron is the cube. As you can see, a cube has 6 flat faces that are polygons — in this case, all of the faces are square — and 12 straight edges. Additionally, a cube has 8 vertexes (corners).
The above figure shows a few common polyhedrons.
The above figure shows a special set of polyhedrons called the five regular solids. Each regular solid has identical faces that are regular polygons. Notice that a cube is a type of regular solid. Similarly, the tetrahedron is a pyramid with four faces that are equilateral triangles.