A polyhedron is a three-dimensional geometric shape that is made up of flat surfaces called faces. Each face is a polygon, which means it has straight edges. Common examples of polyhedra include cubes, pyramids, and tetrahedra. The faces meet at edges, and the points where the edges meet are called vertices. Polyhedra can be classified into two main categories: regular and irregular. Regular polyhedra have faces that are all the same shape and size, such as the five Platonic solids: tetrahedron, cube, octahedron, dodecahedron, and icosahedron. Irregular polyhedra have faces that can vary in shape and size.