The tetrahedron is one of the most common geometries in solid state structures (as well as the vast field of organic chemistry)! It can be thought of as a pyramid with a triangular base, but it turns out that you can choose any of the corners as the tip of the pyramid; they are all the symmetrically equivalent. If you look down certain directions, you will see very symmetrical arrangements. Try looking down on corner toward the triangular base in the back, and you should find a triangular pattern. If you look down a direction in which opposite edges cross each other, a square-like pattern will appear. The tetrahedron is one of the five Platonic Solids, polyhedra in which all of the corners, edges, and faces are the same.