We present a hands-on, inquiry-based activity exploring how CO[subscript 2] input to seawater affects the skeletons of several species of reef-building corals and other marine organisms by testing for changes in pH and calcium ion concentrations. Originally developed to inspire and recruit high school students in the state of Hawai'i into the science, technology, engineering and mathematics (STEM) fields, we assessed the effectiveness of the activity through the responses of 380 high school students to a series of questions about ocean acidification both before and after guiding them through the inquiry-based activity. Our results show that students gain a better understanding of ocean acidification and the relevance of the geosciences to their daily lives after their participation in the activity.