Tom is seven. He goes to school every day. The school is near his home. So he goes there on foot and comes back home on time (准时). But today he is late. His mother asks him, "Why are you late today?" "I'm in the teachers' office. ""Why do you go to the teachers' office?" "Because my teacher asks us a question in class, and nobody can answer it but me. ""It's good to answer the teacher's question. ""But the question is 'Who put the ink (墨水) on my chair?'"