1. There'e two brothers. One lives on one side(边), and the other is on the other side. ______
When people say something, they can hear. But they can' t see each other(互相).  
2. It is black when it's clean and white when it isn't clean.                      ______
3. It lives in a case. When you want to write something, it can help you.           ______
4. It walks all the time, but never(从不) leaves its place.                        ______
5. It can swim in the water. And it has got a tail(尾巴) and fins(鳍).               ______