One day after school the teacher said to his students, "tomorrow morning, if any one of you can answer my first question, I will permit him or her to go home earlier."

The next day, when the teacher came into the classroom, he found the blackboard daubed（涂抹） . He was very angry and asked, "who did it? please stand up!" "it's me," said bob, "now, I can go home. good-bye, sir."