The Mouse, the Frog, and the Hawk explained simply

A Mouse and a Frog struck up a friendship; they were not well mated, for the Mouse lived entirely on land, while the Frog was equally at home on land or in the water. In order that they might never be separated, the Frog tied himself and the Mouse together by the leg with a piece of thread. As long as they kept on dry land all went fairly well; but, coming to the edge of a pool, the Frog jumped in, taking the Mouse with him, and began swimming about and croaking with pleasure. The unhappy Mouse, however, was soon drowned, and floated about on the surface in the wake of the Frog. There he was spied by a Hawk, who pounced down on him and seized him in his talons. The Frog was unable to loose the knot which bound him to the Mouse, and thus was carried off along with him and eaten by the Hawk.