Add food coloring to make the water whatever color you choose. Put the lid on tightly. Make sure the bottle doesn't leak by holding it upside down over a sink. Wash the outside of the bottle with soap and water to remove any excess oil. A brick is more dense than Styrofoam.
Liquids have different densities, too.
The more dense liquid will sink in the less dense liquid and settle to the bottom of the bottle and the less dense liquid will float at the top of the more dense liquid and settle near the top of the bottle.