It's worth noting the close relationship between the C-A-C two-step mechanism and the waterassisted mechanism. Firstly, in the C-A-C mechanism, water interacts with CsPbI 3 by locating at the amorphous nuclei and interfaces. Both DFT calculations and MD simulations have shown that water molecules' intercalation energies in CsPbI 3 crystals are relatively higher than those in amorphous CsPbI 3 (supplementary material and Fig. ), indicating that water facilitates the phase transitions by entering the amorphous areas. The conclusion that water molecules can easily access the amorphous interface is consistent with the experimental observation that phase transition rates show immediate responses to air humidity . Secondly, the mechanism by which water molecules accelerate phase transitions by enhancing interfacial ion diffusion aligns with the diffusion-dominant nature of the C-A-C mechanism . In a phase transition via the C-A-C pathway, the primary factor influencing the phase transition rate (nucleation barrier) is the ion diffusion at the amorphous regions. As a highly polar solvent, water molecules inevitably increase the diffusion rate of interfacial ions, thereby reducing the phase transition barrier.