The Poly Density Kit comes with bottle, white and blue beads and salt. You just need to add rubbing alcohol (70% or 91%). Both work but require different steps to setup.
In the bottle we have a mixture of rubbing alcohol, pure salt, water and white & blue beads.
When the bottle is shaken, the two liquid layers momentarily mix, forming a homogeneous mixture.
Water and rubbing alcohol are soluble in all proportions. Both the water molecules and the alcohol molecules easily hydrogen bonds to each other. The sodium chloride salt particles, however, bind with the water molecules forcing the alcohol molecules out of the water solution.
The white beads with a lesser dense than this liquid mixture float on top and the blue beads with a greater density sink to the bottom. Then, as the salt layer separates from the alcohol, the blue beads rise in the bottom layer and the white beads sink in the top alcohol layer until they meet in the center. From lowest density to highest density the order is as follows: rubbing alcohol, white beads, blue beads, and salt water. Because the beads float between the two liquids, the actual alcohol/salt water interface is difficult to observe, adding to the mystery.