Water and oil don’t mix. The oil is less dense or lighter than water, so it will float above it. Food coloring is as dense as water, so it will sink under the oil and mix with the water. When you drop the antacid tablet, it will sink to the bottom of the bottle where it will start to dissolve, releasing gas and forming bubbles. The colored bubbles will bring along some water as they rise to the top of the bottle, until they pop. Then the colored water will once again sink to the bottom of the bottle. This will happen repeatedly until the tablet dissolves completely.