Water demons in Japanese mythology.  They live in ponds and try to drown people (mostly children) who wander too near.  They say the way to defeat a Kappa is to bow when meeting one, because it tends to copy the gesture.  A Kappa's head is bowl-shaped and is filled with water where it derives most of its power.  When it bows, the water spills and the kappa loses its power.

Or, something along those lines, I think.