What You Do:
1. Fill the flask most of the way with vegetable oil.
2. Fill the rest of the flask with water. The water will sink to the bottom under the oil.
3. Add a few drops of food coloring; your choice of color. The food coloring is water-based, so it will also sink and color the water that is now at the bottom of the flask.
4. Break an alka-seltzer tablet into a few small pieces, and drop them in the flask one at a time.
5. Watch your lava lamp erupt into activity! As the reaction slows down, simply add more alka-seltzer.