Make an oil slick with oil and cocoa powder, then attempt to clean it with a string boom and cotton ball skimmers.
Science content (2016 curriculum):
Chemistry: Physical Changes, Solutions, Mixtures and Separating (2, 4, 5, 6)
Earth/Space: Water cycle, Water conservation (2)
Earth/Space: Sustainable practices, Interconnectedness (2, 5, 7)
Science topic (2005 curriculum connection):
Earth and Space Science: Renewable and Non-Renewable Resources (grade 5)