A tornado sweeps through the plains of Kansas, lifting Dorothy and Toto. The two tumble into Oz, landing on the Scarecrow. After freeing him from his pole, the trio stroll together, soon finding a Tin Woodman and oiling him.
After the four watch mating rituals of various animals set to strains of Camille Saint-SaĂŤns's "The Swan", they are welcomed into the Emerald City. Suits of armor sing to them, "Hail to the Wizard of Oz! To the Wizard of Oz we lead the way!" A creature resembling the A-B-Sea Serpent of The Royal Book of Oz extends itself as stairsteps for Dorothy to enter the coach.
The Wizard is a cackling white-bearded man in a starry black robe and conical hat who produces custom seats for each of the four nervous travelers, including one for Toto (the Toto chair is mostly cut out of the frame in most video versions, but is later shown in a full shot of Toto sitting). He proceeds to perform magic with a hen and eggs. These are variations on simple sleight of hand tricks involving making objects appear, but the hen is able to take the eggs back into her body.
Finally, the hen releases an egg that will not stop growing. The five try to fight it, with the Tin Woodman breaking his axe. Soon, though, the egg hatches, the hen takes the chick, and clucks out "Rock-a-bye Baby" as a chorus joins her. The five laugh, and the film ends on an iris-in of mother and child.

Question: What does the hen do?
Answer: She releases an egg that won't stop growing.
Alternative answer: take the egg back into her body