The United Nations has proclaimed 2011 as the International Year of Chemistry and, as part of that designation, has invited teachers worldwide to participate with their students in the Global Water Experiment (IYC 2010). Students have the opportunity to take part in four different water-related investigations to assess water quality and then share data with other students around the world. The project began on World Water Day (March 22 each year) and will run throughout 2011. This article looks at a related water exploration--designing charcoal filtration systems like those used in filtering water pitchers, some coffee pots, and refrigerators. In this 5E learning-cycle activity, students will investigate how charcoal (carbon) filters can be used to remove chlorine from water. They will design a filtering system to reduce the chlorine concentration in a sample of water while also minimizing the amount of charcoal that is required. (Contains 5 figures and 4 resources.)