I think this may be one of my favourite science activities. I originally found it on Steve Spangler's site. With my buddies, I like using test tubes over bottles because it cuts down on the amount of oil needed and gives a wonderful effect. We've done it many times, and are due to do it again, I think. This video shows my darling granddaughter doing the experiment!
All you need is cooking oil, water, food colouring and alka seltzer tablets - we broke ours into bits. Adjust the amounts of your ingredients to the size of your container. It should be mostly oil, a smaller layer of water and a bit of colouring. What we did was add a few tablespoons of water, a few drops of food colouring and about 1/3 cup of oil to our test tube, then dropped in some alka seltzer tablets and watched the action. It's mesmerizing!