Consumers want to know what is in the water coming out of their taps. But there is a huge variety of water-quality tests on the market, which can be daunting for the average person. At the ACS national meeting, Rebecca H. Lahr, a civil and environmental engineer at Michigan State University, presented a simple, low-cost, “open source” water test. Her method is based on the unique residue patterns formed by water droplets as they dry. This phenomenon, called the coffee-ring effect, occurs as the water evaporates, moving particles within the droplet to its outer edge and leaving behind residue that resembles a stain created by spilled coffee. Users simply place a drop of water on a piece of polished aluminum sheeting. After the water evaporates, they snap a photo of the remaining residue using a jeweler’s magnifying glass and a cell phone camera. Lahr’s team is creating a library of residue patterns for various tap water components. Users could then compare the patterns they see to those in the library to identify what is in their water. Because this method is so straightforward, Lahr hopes teachers will use it in classrooms to inspire curiosity about chemistry and tap water. A quick, cost-effective test for baseline water quality could also be useful for other scientists and utility managers, she said.