Hydrogels Water purification is an energy-intensive, multistep process used to remove contaminants such as salt, heavy metals, oils, or biological pathogens. Xu et al. designed a hydrogel that purifies water using nothing more than sunlight. Polydopamine, a melanin-based polymer, was added to sodium alginate on top of a microporous poly(N-isopropylacrylamide) (PNIPAm) hydrogel. Upon immersion in water, the hydrogel swells up with water while rejecting the contaminants, in part enhanced by the sodium alginate. Sunlight absorbed by the polydopamine forces the PNIPAm to heat up above its lower critical solution temperature, causing it to collapse, become hydrophobic, and expel the clean water. The process can be cycled, as the authors showed in a field test using lake water. Adv. Mater. 10.1002/adma.202007833 (2021).