This practice insight examines how science mediators facilitate students' appropriation of scientific knowledge. It focuses on the Science of Superheroes initiative, which is an informal education curriculum activity designed to engage middle school students in exploring environmental challenges through the creation of a superhero. Using Bruner's scaffolding framework, we analyze how mediators' interventions shift between scaffolding functions, depending on the task. While the appropriation of scientific knowledge is supported by questioning and information-sharing strategies, the creative phase sees an increase in proposal-based scaffolding, which struggles to counterbalance students' reliance on magical thinking. This cognitive tension highlights the challenges of integrating scientific knowledge into a fictional narrative. Our findings highlight the need for science mediators to refine their questioning techniques, foster greater self-regulation among students, and enhance their ability to meaningfully incorporate scientific concepts into their superhero designs. Our study contributes to ongoing discussions on the professionalization of science mediation and offers new insights for mediator training.