Mario hitting Bowser's Funny Bone.
The Funny Bone is an area in Bowser's Body, apparently located in Bowser's spine. When the mario bros. first get here, Starlow goes before them to investigate and discovers that one of the bones in the area is the center of Bowser's nerves. After knowing this, Mario hits with a hammer the bone, making Bowser to wake up fom his knocked-out state.