What the trolls had done

After a few steps, the boy started to talk to him. "It's a good thing you don't carry me as badly as you carried the changeling" he said. "What do you mean?"
"Well, the troll woman was walking on the other side of the ravine with me in her arms; every time you stumbled and almost dropped the changeling, she stumbled and she almost fell with me."
"What are you saying?" Were you walking on the other side of the ravine?" asked the farmer, and grew thoughtful.
"I've never been so frightened in my life" said the boy. "When you threw the changeling into the ravine, the troll woman wanted to throw me after. If mother hadn't ..."

The farmer began to walk more slowly, while he questioned the boy. "Tell me how you were treated with the trolls."
"Sometimes it was hard." said the little boy. "But so long as mother was kind to the troll child, the troll woman was gentle with me."

"Did she beat you?" asked the farmer.
"She never beat me more than you beat her child."
"What were you given to eat?"
"Each time that mother gave the troll child frogs and mice, I was fed with butter and bread. But every time you offered the troll child bread and meat, the troll woman offered me snakes and thistles. The first week, I almost starved to death. If mother hadn't — "

When the child said this, the farmer turned on his heel and started to walk briskly back to the valley. "I don't know how it's happened, but you smell of smoke and fire."
"Well, that's not surprising", said the child. "I was thrown into the fire last night, as you threw the troll child into the burning farmhouse. If mother hadn't — "

The farmer now was almost running back down the hill, but a thought pulled him up short. "Now you must tell me why the trolls have freed you", he said.
"At the moment when mother offered something worth more than her own life, they lost their power over me, and must release me" said the boy.
"What did she sacrifice, that she loved more than life itself?" asked the farmer. "She did that when she let you go, so that she could save the troll child." answered the boy.