How do I make my own liquid wave motion toy?

I'm not entirely sure what this toy is called. It's a novelty item usually sold next to the lava lamps.





It's a clear, sealed, rectangular container containing two different liquids. The top layer is clear, and the bottom layer is usually colored blue.





When you rock this container back and forth, an cool-looking, slow-motion wave forms inside. It's entertaining and relaxing to watch. Sometimes, the toy has a motor that slowly rocks the gadget back and forth so that the wave is always going.





What are the two liquids used to make this toy? Is it as simple as oil and water? What kind of dye is used so that it can color one liquid but won't color the other?|||Oil and water should work. You can buy lamp oil, which is already decoratively colored and would end up being your top layer. You should be able to add food coloring to the water and make it whatever color you want. Good luck!