Detailed Instructions: In this task, you're given an article and an answer. Your task is to generate the question for the answer based on the given article.
Problem:Article: John sent his mother expensive presents on his mother's birthday every year. He liked to show her how rich he was. "I must find something wonderful for her, and it must be the only one in the whole world. Then my mother must be very happy." John thought. He advertised in the newspapers. "Wanted--the best present for the woman who has everything. " For many days his telephone did not stop ringing. People phoned him from all over the world. They wanted to sell him "the best present". But they did not have anything that was the only one in the whole world. Then less than a week before his mother's birthday, a man came to his office. He was carrying a cage, and in the cage there was a large bird. "This bird, "the man said, "can speak ten languages and sing any pop song. There is no other bird like it in the world. " "I will listen to it, "John said. "If what you said is true, I will buy the bird from you." The man spoke to the bird. "Talk to me in French, " he said. The bird spoke to him in French. "Tell me a joke in Japanese, " he said. The bird told him a joke in Japanese. "Sing a famous pop song, "he said. The bird sang a famous pop song. "I'll buy it, "John said. "How much do you want?" "One hundred thousand dollars, "the man said. This was a lot of money but John paid him. Then he sent the bird to his mother with a birthday card. The day after his mother's birthday he phoned her. "Well, mother, "he said, "Is the bird wonderful?" "Oh, it was delicious, dear, "she said.
Answer: Because he was the richest man in the world.
Solution:
Why did John send her mother expensive presents for her birthday every year?