2. A triangular pyramid, geometric, consisting of four triangles. There is one vertex when the bottom surface is fixed and four vertices when the bottom surface is not fixed. A regular triangular pyramid is different from a regular tetrahedron, and there must be a regular triangle on each face of the regular tetrahedron.
3. The polygon on the plane has at least three sides, and the geometry of space has at least four sides, so tetrahedron is the simplest geometry of space. Tetrahedron is also called triangular pyramid. A triangular pyramid has six sides, four vertices and four faces. A triangular pyramid whose base is a regular triangle and whose vertex is the center of the bottom triangle is called a regular triangular pyramid. A tetrahedron composed of four congruent regular triangles is called a regular tetrahedron.