Soddy spheres, or tangent spheres, is a recursive space filling method. Each sphere is touching at least four other spheres, including the spherical space the tiny spheres are packed in. See http://mathworld.wolfram.com/TangentSpheres.html
The model contains of 81 spheres with the following diameters: 4x45mm, 4x29mm, 12x21mm, 24x14mm, 12x12mm and 25x10mm. These are packed in a spherical space with a diameter of 100mm.