One peculiar trait is that it has a cavity on its head called a sara (“dish”, “bowl”, or “plate”) that retains water or some sort of liquid, which is regarded as the source of the kappa’s power or life force. This cavity must be full whenever a kappa is away from the water; if it ever dries out, or if its water is spilled, kappa loses its power and may even die.[9][10][8]

Source: Kappa (folklore) – Wikipedia