Water plays a crucial role in the B↔Y phase transition in MHPs . Under dry conditions, rapid cooling can maintain the material in the metastable black β and γ phases without transitioning to the yellow phase . Experimental evidence suggests that without water molecules, defect introduction via laser irradiation is insufficient to induce the B→Y phase transition . However, under humid conditions, water molecules in the air trigger the nucleation process of the B↔Y transition, and the transition is highly sensitive to environmental humidity . As humidity increases from 53% to 73%, the nucleation barrier of yellow-CsPbI 3 decreases from 1.34 to 0.57 eV . Additionally, the rate of the B→Y phase transition in CsPbI 3 nanowires exhibits an instantaneous response to environmental humidity, enabling real-time control of the transition rate by regulating environmental humidity . Differential scanning calorimetry experiment has confirmed that the enthalpy change of the B→Y phase transition in CsPbI 3 is independent of water content, suggesting that water molecules catalyze the phase transition process rather than forming H 2 O adducts .