"The Dirty Water Challenge" is a fun activity that teaches children about their environment in an engaging and practical way. Inquiry is embedded within the practical--students have to design, plan, and then build their own design of water filter. Students are exposed to important concepts from a variety of scientific disciplines, including how the water cycle works (geology, meteorology) and the principles behind water filtering (physics, chemistry). This method has been successfully used during units on the water cycle and pollution to teach elementary and lower middle-school classes in an inquiry-based way. (Contains 1 figure, 1 resource and 2 online resources.)