Brothers Grimm 
Frog Prince 

The princess lost her favorite toy. The frog promised to help her find it back and asked the princess to promise to be the good friend. The princess agreed. But when the frog handed the golden ball to the princess, the princess broke her promise because the frog was ugly. When the king knew about it, he asked the princess to abide by her promise. The princess had to accept the frog»s requests. The frog became a prince and lived a happy life with the princess. This story tells us to be honest and trustworthy.