Professor Li was on a hotpot these days. He had difficulty in communicating with his son. Professor Li had stayed in America for more than 10 years and just came back to China with his family. His son was 14 years old, and recently Professor Li found his son always locked himself in his room and said nothing about what he was doing. Professor told his friend his problem, and his friend asked why he did not open the room's door to see what happened. Professor said he couldn't because that was his son's room, so he could not open the door without his son's permission.