A new, simple, and inexpensive method that uses ultraviolet light to control particle motion and assembly within liquids could improve drug delivery, chemical sensors, and fluid pumps. The method encourages particles—from plastic microbeads, to bacterial spores, to pollutants—to gather and organize at a specific location within a liquid and, if desired, to move to new locations. A paper describing the new method appears in the journal Angewandte Chemie.