1．When does a student's brain stop working in class？
学生在课堂上什么时候大脑停止工作？

2．When can Mother stop worrying about her delicate vase？
母亲什么时候才不会担心她那只精致细巧的花瓶？

3．What can't talk but can speak the truth？
什么东西不会讲话却会以实相告？

4．What's full when it's used and empty when it's at rest？
什么东西用时满，闲时空？

Keys：
1．When they're called on to answer questions．
当叫到他们回答问题时。

2．When it's broken．..
摔破以后。

3．A mirror.
镜子。

4．A shoe.
鞋子。