You will need: water, vegetable, olive, or some other kind of oil, food coloring and a clear glass. (bonus points if you wear goggles. They're totally not necessary, but very cool nonetheless)
Start by coloring your water any color you want, I used blue. Pour about two inches of oil in the bottom of your container, then add another two or three inches of your colored water. Allow this to settle for just a second, then add an Alka Seltzer or denture tablet and sit back and enjoy the show.