Read the following German fairy tale.
Hansel and Gretel
Hansel and Gretel lived with their father and stepmother. Because the family had little to eat, their cruel stepmother decided to lead the children into the forest and leave them there.
Hansel and Gretel wandered, hungry and tired, until they came to a house made of gingerbread and candy. The children began eating pieces of the house. Suddenly, an old woman came out. She said, "Come inside and you will have all the sweets you like."
But the next day, the woman locked Hansel up, and she made Gretel do all her work. "When Hansel is nice and fat, I will eat him for supper!" said the old woman, who was really a witch. Every day she asked to feel Hansel's finger to see if he was fat yet. But clever Hansel held out a bone, so the witch, who couldn't see well, thought he was still thin.
Time passed, and the witch decided to eat Hansel anyway. She told Gretel to start the fire in the oven, but Gretel had an idea. "I can't see if the fire is going," Gretel said. When the witch looked in, Gretel pushed her into the oven and locked the door. Then she and Hansel escaped.
What is the main theme or lesson of the fairy tale?