The 5 Platonic forms are an important aspect of sacred geometry. The Greeks considered them to be the only perfect straight edged geometries, and they associated them with the 5 elements.

They can teach us about the basic symmetries of space. The tetrahedron is the simplest (and also called the 'simplex') but needs its interpenetrating dual to complete it, making a 'star tetrahedron'. The corners of the star tetrahedron form a cube, and the intersection of the two tetrahedra form an octahedron. 

5 cubes can be arranged to form the vertexes of a dodecahedron. And the dodecahedron's edges can be extended to form the vertexes of an icosahedron. This shows one way in which all 5 forms related.