Once upon a time, there was a school that was so dirty. There were many rats and other insects. That made the students feel very uncomfortable. One day, the students who study in that school felt that they did not like the school's atmosphere, so, they decided to change it. Some of them were in charge of sweeping the floor, some were in charge to do something else. Therefore, all of the students were working very hard to clean their school. When they had finished their work, one student shouted out," Everyone, let's keep our school as clean as this everyday!". And everyone said yes. So, from now on, they clean their school once a week.