The Frog King or Iron Heinrich by Brothers Grimm

"a golden ball, throw it into the air, and then catch it"

" 'i can help you, but what will you give me if i bring back your plaything?
whatever you want, dear frog'
'if you do love me and accept me as a companion and playmate, and let me sit next to you at your table and eat from your golden plate and drink from your cup and sleep in your bed, if you will promise this to me, then ll drive down and bring your golden ball back to you'
'oh yeah i promise' "

"youngest daughter of the king,
open the door for me,
don't you know what yesterday,
you said to me down by the well?
youngest daughter of the king,
open up the door for me
what you have promised, you must keep.  Go and let the frog in"


"you should not despise someone who has helped you in time of need"


In some variants the moral of the story is not just "Beauty is only skin deep" 
but instead some of the variants have a second moral. In this particular
variant there is an underlying moral of both keeping your promise as well
as being kind to those who have helped you; you just never know when
you may need their help again. in this story the princess is helped by a
slimy frog whom she is terribly disgusted by. instead of showing her thanks
to the frog, his appearance causes her to shun and avoid him. The father
tells her to be nice but she can not find it in her. In the end it turns out that
despite the frogs warm heart and helpful personality, his looks do just not
meet the standards of the princess. She is cruel toward the frog because of
this. It turns out that the frog is not just a frog but a handsome prince
stuff in the slimy creatures body. Showing just how little you know about
someone just by looking at their physical characteristics.

"threw him against the wall with all her might.  'Now you will have your peace you disgusting frog!' "