There was a boy and his family was very rich. One day his father took him on a trip to a place. He wanted to show his son how poor people were there. They spent several days on a farm. There were many poor families living on the farm. On their way home, the father asked the son, "How was the trip?" "It was great, Dad," the son answered. "Did you notice how poor people were there?" asked his father. "Oh, yes," said the son. "So, tell me," said his father. The son said, "Well, we have one dog and they have four. We have a pool in our garden, while they have a river that has no end. We have expensive lanterns  , but they have stars above their heads at night. We have only a small piece of land to live on, while they have the endless fields. We buy our food, but they grow theirs. We have high walls around our houses to protect us, but they don't need walls, because their friends will protect them." The father had nothing to say. Then the boy added, "Thank you, Dad, for letting me see how poor we are." How often did they go to the farm?
Once a week. Twice a month. Every year. The writer doesn't tell us.