This article reports on a new device, called a geyser guide, that allows for quantitative measurements of both bubble sizes and kinetics of foam production in the so-called Diet Coke and Mentos experiment. The device is easily constructed, is made using readily available materials, and allows the experiment to be carried out indoors with no mess. In contrast to previously reported measuring devices used to monitor this experiment, the geyser guide allows for tall columns of foam to be produced and visualized. This feature makes experiments with the geyser guide suitable for demonstrations in large classrooms. Investigations with the geyser guide can be tailored for use in settings that span from elementary science classrooms to undergraduate research projects. Explorations with the geyser guide have allowed for estimates that 2.4-14 million bubbles per liter of Diet Coke are produced during the experiment and that a single Mentos candy contains 50?000-300?000 nucleation sites.