Inspecting S states in photosynthesis: Oxygenic photosynthesis uses a Mn ₄ CaO ₅ cluster in the oxygen-evolving complex to extract electrons from water and produce dioxygen. Visualizing each of the chemical states in this process, S ₀ to S ₄ , and assigning chemical identities and mechanisms on the basis of structures has been a challenge addressed recently by work at x-ray free-electron lasers. Suga et al. used serial crystallography at cryogenic temperatures to trap and determine the structures of several stable states during photosystem II water oxidation (see the Perspective by Britt and Marchiori). Changes around the water cluster already happen in the S ₂ state and set the stage for water insertion that occurs during transition to the S ₃ state. A short 1.9-angstrom distance between the two oxygen atoms in the S ₃ state is consistent with theoretical studies supporting an oxyl/oxo mechanism for oxygen-oxygen coupling. Science , this issue p. 334 ; see also p. 305