Density columns are made by layering liquids of different densities. If neighboring layers are insoluble with each other, the column will stay in separate layers unless disturbed.
The problem arises when the layers are mixed and exposed to other layers with which they are soluble. If this happens, the soluble layers will combine and usually end up settling into two layers: a hydrophilic (polar) watery layer and a hydrophobic (nonpolar) oily layer.
The Klutz-Proof Density Column uses a phenomenon called “salting out” to preserve its layers. When salt is added to the mixture, the ions bond with the water molecules and exclude the slightly less polar alcohol molecules from forming hydrogen bonds with water, making it no longer soluble with water. The alcohol stays separate and ends up as its own layer on top of the column because it has a lower density than oil.
Since the salt causes all three liquids to be insoluble with each other, you can mix the column as much as you like—it will always settle into three layers. This makes it the perfect density column for klutzes.