The roof-cleaner

Every morning at exactly half past eight the old crow starts his work. He comes to the roof of the building accross the street and begins. He inspects the roof-tiles, knocking on those that might be loose. He keeps the roof clean, too. If he sees rubbish that doesn't belong there, he picks it up, pushes it under a tile and then knocks on the tile firmly, making sure that the dirt doesn't get out. He inspects the whole roof like this every day. He never forgets.