Although residues and resource shortages are major problems of modern living, these topics are not usually properly covered in university curricula. To tackle this situation, we have guided undergraduate students through the design of a group of active learning activities, to demonstrate the science behind separation and recycling: trash separation and recycling of paper, plastic, oil, and metals (lithium and cobalt available in mobile phone batteries). With a better understanding of the underlying scientific processes and fundamentals, undergraduate students were engaged to undertake and develop adequate physicochemical procedures for waste separation and recycling. A selection of the activities was presented at a science and technology fair at the University of the Balearic Islands as outreach activities that enrolled undergraduate students acting as teachers for fair visitors. The results showed a good engagement rate of the involved undergraduates as well as a stimulated visitors' interest.