THE STRANGER fills the frogs belly with quail shot.
The title of the book is 'The celebrated jumping frog of Calaveras county'.
Smiley bet with a stranger that his frog can beat any other frog and the stranger agreed to go into competition with him. Smiley lost the competition, because unknown to him the stranger has filled his frog with quail shot when he went to the stream to catch a frog for the stranger.