Parents won't get you a lava lamp no matter how much you ask for one? Make your own. No electricity required -- just the concept of density.
What's Going On?
As we mentioned, water (and food coloring) is denser than the oil. This means that the oil, which is "lighter", floats on top. When the alka-seltzer is dropped in, it causes a reaction to take place and creates carbon dioxide bubbles. These bubbles of gas stick to the water droplets, making the water less dense than the oil, unlike how it was before. This causes the water to rise up, creating the "lava lamp" effect. As the carbon dioxide gas bubbles reach the surface of the fluid, the gas bubbles are released into the air, causing the water to become more dense and sink to the bottom once again. This process is repeated again and again until carbon dioxide levels start to decline and (and then you add more alka-seltzer to do it all over.)