This study examines the impact of a mobile game app on science museum visitors' level of engagement with exhibit content, compared to a non game-based version of the same app. Ask Dr. Discovery (Dr. D) is a question-asking app containing two versions: a Game Mode employing casual game mechanics and an Ask Mode providing a baseline version. We implemented both versions of Dr. D at two science museums located in the Southwestern United States with 1539 participants. In both conditions, participants could type or speak questions to receive vetted answers about museum content, but only Game Mode embedded question-asking within a simple game. Participants' level of engagement was represented by the number of questions asked about exhibit content in Dr. D. Additionally, we explored the relationship between app engagement and participants' self-reported level of interest in science. All participants completed pre- and post-questionnaires with questions related to science interest, impressions of the Dr. D app, and demographic information. Results in both museums indicated that users of Game Mode asked nearly twice as many questions on average as participants using Ask Mode. Science interest predicted engagement at one of two sites. Demographic variables, including gender, age, and race/ethnicity were not found to influence the rate of question asking in either mode. These results indicate that employing simple game mechanics in apps for museum visitors may lead to strong positive impacts on visitor engagement with museum content.