Tetrahedron

A tetrahedron, also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges and four vertices, and is the simplest of all ordinary convex polyhedra. It is the three-dimensional case of the more general concept of a Euclidean simplex, so it may also be called a 3-simplex. As a pyramid, it is a polyhedron with a flat polygonal base and triangular faces connecting the base to a single apex; because a tetrahedron's base is itself a triangle, and any of its four faces can be treated as the base, it is also called a triangular pyramid. Like every convex polyhedron a tetrahedron can be folded from a single sheet of paper, and it has two distinct nets for doing so. Every tetrahedron has a circumsphere passing through all four of its vertices and an insphere tangent to all four of its faces.