This quantitative survey study aimed to identify “active ingredients” of a science festival in The Netherlands. Active ingredients are the elements of science communication activities that drive the impact on visitors' knowledge, attitudes, or behavior. Factor analyses of data from on-site surveys conducted in two different festival years (Total N=456) revealed three active ingredients: personal relevance, accessibility, and interactivity. Furthermore, the analyses revealed two impacts: increased knowledge/insight and increased familiarity with science. The strongest predictor of impact was personal relevance, which denotes the feeling that the festival activities touched on visitors' emotions and personal life.