What if a material’s history mattered more than its composition? Using in situ neutron diffraction (D20, ILL) to watch a strontium-rich Ruddlesden–Popper oxide being reduced, this study shows that a single precursor can become either a high-performing fuel-cell anode or a robust methane-reforming catalyst — depending solely on how it is activated. By tracking, in real time, the oxygen that X-rays cannot see, neutrons turn this « activation engineering » into a predictable design tool.