The increasing generation and improper disposal of urban solid waste pose a significant challenge to Sustainable Development Goal 11 (SDG 11), specifically Target 11.6. Inadequate waste separation reveals gaps in environmental education, demanding innovative pedagogical strategies. This work presents the development of "Lixeira Certa", a mobile serious game based on Game-Based Learning (GBL). The design features an arcade-style drag-and-release (slingshot) interaction where players sort waste into bins under time pressure. Unlike passive methods, the core loop prioritizes motor interaction and procedural memory. The prototype integrates a scaffolding mechanism with visual feedback to reinforce cognitive association. By aligning immediate feedback with SDG 11 goals, the project offers a scalable technology-driven approach to child environmental awareness.