Fill a clear bottle or jar about three-quarters full with water
Add food coloring to the water if desired
Pour vegetable oil into the container until it is nearly full
Wait for the oil and water to separate into layers
Break an effervescent tablet into small pieces
Drop one piece into the container
Watch the bubbles rise and fall through the liquid
Add more tablet pieces as the bubbling slows
Seal the container only if it is not being used for observation
Use a flashlight or lamp underneath or behind the container for a stronger effect