Fill the balloon with water . This image illustrates why you don't completely fill the balloon you should always leave some room in the neck of the balloon, then as the form dries the pressure has somewhere to go and your clay wont split like that.
Acorn Art - flickr lilfishstudios: whole lineup of tiny acorn ornaments I've made. I hollowed out acorn bodies and glued in tiny mushrooms I made out of wool. Crafts for squirrels