A kappa's power lives specifically in the water it carries in a shallow depression on the crown of its head, and nearly every account of outwitting one turns on getting at that water rather than on any weapon or exorcism. A common motif has a kappa challenging a passerby to a wrestling match or attempting to pull them into the river; if the human bows deeply, the kappa, bound by rigid etiquette to return a bow, bends its own head forward and spills the water out, instantly losing its strength and becoming harmless or even helpless until the water is somehow restored. The same vulnerability appears in stories of kappa captured after being tricked into spilling their water by accident, after which the creature, now powerless and at a human's mercy, is described bargaining for its release, sometimes offering to teach the family a form of bone-setting medicine in exchange, a skill several rural Japanese families traditionally claimed to have inherited from exactly such an encounter.
The bow trick and the bargaining stories together give the kappa a distinctive folkloric character among Japanese water hazards: a creature genuinely dangerous to swimmers and livestock, blamed traditionally for drownings, but one whose danger is governed by rules a clever or polite human can exploit, rather than a purely hostile force with no exploitable weakness. That combination, real threat paired with a fixed, learnable vulnerability, is part of why the kappa remains one of the most recognizable and frequently retold yokai in Japanese popular culture today, well beyond its original role as a rural cautionary figure warning children away from unsupervised water.