One day, Mr. Li was reading a book in his office after school. To his surprise, a ball flew in suddenly and hurt him on the head. He took the ball to the window, but he only saw a group of boy students running away quickly and disappearing in a minute. He sat down again and continued his reading. As it got dark, it was time for him to leave for home. Looking at the ball, he wondered what to do with it. He knew the owner of the ball must be nearby and wished to get it back. He went downstairs and walked to the playground. It was at the center of the playground that he placed the ball with a piece of paper, on which he wrote “Be careful next time！”. Then he walked away.
The next morning, as Mr. Li opened the office door, he found the same piece of paper on the floor. Some words were added after his words: “Thank you for your kindness. Anyway, you are a clever fellow.”
( )1. What happened while Mr. Li was reading in his office
A. The window was broken by a ball.