我国著名化学家傅鹰告诫我们“化学给人以知识，化学史给人以智慧。”回顾苯的结构的探

     Even though it was only October, my students were already whispering about Christmas plans. With each
passing day everyone became more   1   waiting for the final school bell. Upon its   2  everyone would run for
their coats and go home, everyone except David.
     David was a small boy in ragged clothes. I had often   3    what kind of home life David had, and what kind
of mother could send her son to school dressed so   4    for the cold winter months without a coat, boots, or
gloves. But something made David   5  . I can still remember he was always   6   a smile and willing to help. He
always
  7   after school to straighten chairs and mop the floor. We never talked much. He   8   just simply smile and
ask what else he could do, then thank me for letting him stay and slowly   9   home.
     Weeks passed and the  10   over the coming Christmas grew into restlessness until the last day of   11 
 before the holiday break. I smiled in  12   as the last of them hurried out of the door. Turning around I saw
David   13   standing by my desk.
     "I have something for you," he said and  14   from behind his back a small box.   15   it to me, he said
anxiously,"Open it. " I took the box from him, thanked him and slowly unwrapped it. I lifted the lid and to my
 16   I saw nothing. I looked at David's smiling face and back into the box and said, "The box is nice, David,
but it's  17  ."
     "Oh, no. It isn't." said David." It's full of love. My mum told me before she died that love was something
you couldn't see or touch unless you know it's there."
     Tears filled my eyes  18   I looked at the proud dirty face that I had rarely given  19   to. After that
Christmas,David and I became good friends and I never forgot the meaning  20   the little empty box set on
my desk..