What happen to my son

　　In autumn, a father took his son to a small tree. He measured his son and knocked in a nail as a mark. He hoped the boy could go taller in the next year. In winter, Its snowed very hard. However, the nail was still in trunk of tree all the time. Spring came, and the small tree grew stronger. Father brought his son to the tree again. He was eager to see how much the boy had gone since last year. To his surprise, he found that his son was shorter than the mark. He wondered what happened to my son?