Once you've added the rubbing alcohol, you've completed your density column! If you'd like, you can now test the density of a variety of other materials by seeing how far they sink! How far does a peanut sink? How about a cherry tomato, grape, or raisin? Feel free to vary the individual layers as well! How does vinegar, or molasses compare to your other liquids?
How it works:
Density is defined as an object's mass, per unit volume, or, more generally, how compact a material is. An anchor is more dense than water, which is why it sinks. An air bubble is less dense than water, which is why it rises to the surface. And a balloon filled with Helium is less dense than air, which is why it will float away! Each layer in your column contains a liquid with a different density, which gives you the cool layered effect you observed!