Materials:
Large empty soda bottle
Vegetable oil
Food coloring
Water
Alka-Seltzer or similar effervescent (fizzy) antacid

What To Do:
First, fill the soda bottle about 3/4 of the way full of vegetable oil, then fill it (almost) the rest of the way with water. Add about 10 drops of food coloring in the color of your choice, and finally add the antacid tablets.

The layer of oil floats on top of the layer of dyed water because the oil is less dense than water. They layers don’t mix because oil is a non-polar liquid, while water is polar, meaning that it is very difficult to dissolve one in the other. When the Alka-Seltzer is added, it begins to dissolve in the water, releasing buoyant bubbles of carbon dioxide. These bubbles rise quickly to the surface, dragging blobs of colorful water with them. When the bubbles burst at the surface, the water sinks once again below the oil.

(Real lava lamps work on a slightly different principle, but still employ differing density. The colorful wax that makes up the "lava" in a standard lava lamp is denser than water at room temperature, so it rests at the bottom of the water in the lamp while it is turned off. When you switch the lamp on,  the wax is heated by a light bulb in the lamp's base. As the wax warms, it becomes slightly less dense than water, so it rises to the top of the lamp. There the wax cools down and sinks again, creating the mesmerizing cycle for which the lava lamp is famous.)

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