Many recipes for elementary science activities suggest making carbon dioxide from baking soda and vinegar; however, they often do not give exact measurements of the ingredients. The author was able to turn this "drawback" into a plus by challenging her fifth-grade students to find the "ultimate fizz"--i.,e., "What amount of baking soda added to a set amount of vinegar gives the maximum reaction without having leftover baking soda?" As students investigated this question with enthusiasm, graphing and measurement skills developed in the process. (Contains 2 figures.)