Oil and water don’t mix because of how their molecules are constructed. Water is what is known as a polar molecule. Vegetable oil, on the other hand, is a nonpolar molecule. Food coloring is a polar molecule because it dissolves in water.
Here’s the surprising part . . . the Alka-Seltzer tablet reacts with the water to make tiny bubbles of carbon dioxide. These bubbles attach themselves to the drops of colored water and cause them to float to the surface. When the bubbles pop, the color drops of water sink back to the bottom , and the whole thing starts over until the Alka-Seltzer is used up. When the chemical reaction between the Alka-Seltzer and water is over and the bubbling stops. Store it and use it at a later time.
WARNING: Never put the cap on the bottle while the Alka-Seltzer is still making bubbles as it is building up pressure. Once the Alka-Seltzer is completely done then you can put the cap back on for safekeeping.