This is a very interesting class of objects based on a physical phenomenon of the dense spatial package of spheres. The parts, of these puzzles, are actually chains of spheres coupled rigidly and placed within one plane (important from the point of view of a molding design).
The oldest toy of this class known, is called CHEOPS PYRAMID. It is a set of four part, of 2 pairs. Each pair is a different type of part. The task of the player is to build a thrihedral pyramid - tetrahedron 4x4x4. Judging by the shape of the parts the problem is nontrivial, though not very complex:

This game has an essential disadvantage: the pyramid is assembled by stacking of the parts without mutual clutching and goes to pieces with the slightest push.
All the toys suggested, having preserved all the advantages of the prototype, surpass it by the entertainment degree and do not possess the drawback mentioned: they assemble with the mutual part clutching.

Here are some of our THE MOLECULAR PUZZLES:

If the parts are metallized, the amazing optical phenomenon of mutual reflections in contacting spheres may result. This phenomenon is extensively applied for decorating lawns, (for example, see Scientific American #2 1989). Mirror puzzles will be extremely attractive.

Thus, THE MOLECULAR PUZZLES are extremely simple technology, have low prime cost plus high consumer's effect.