Weird story. And nice topic.
There's a type of water demon called a Kappa (河童):
They're very strong. They have a sort of bowl on their head to store water in, from which they derive their incredible strength. To kill a Kappa, you must bow, and being a very polite creature it will bow back, and the water will run out, therefore the Kappa becomes seriously weakened.
They're very mischevious, they do things such as stealing crops, kidnapping children, and raping women. They like eating small children, but they will eat adults too.
(More information on
Wikipedia.)