Technicolor Fog Factory by Bryce Hixson
Drop a chunk of dry ice in warm water, and it will sublimate from a solid to a gas. The gas bubbles boil up from the bottom of the container and, when they hit the air, burst into a small pile of fog. When bromothymol blue is added to the mix, the color of the water changes from blue to sea green to yellow as the carbon dioxide from the dry ice changes the pH of the solution. Pour in some sodium hydroxide, and the color reverts to green and then blue.