Fill a tall bottle most of the way with water.
Place about a tablespoon of cooking oil in a cup. Add a few drops of different colors of food coloring and stir them into the oil as best you can. (Limit yourself to one or two colors of food coloring - I used a bunch and it just ended up looking black - not what I was going for!)
Pour the oil onto the water in the bottle and observe.
There are a few lessons to be gleaned from this fun demonstration:
--Oil is less dense than water (i.e. it floats on the water)
--Food coloring is a polar substance, oil is non-polar (food coloring doesn't dissolve in the oil)
--Food coloring and water are both polar substances (food coloring dissolves in the water)