Charlie studied in a famous college for four years．He studied hard and did well in all his subjects．He hoped to become a good teacher．This year he left the college and began to work in a senior high school．He likes his students and is strict with them．He does his best to make his classes lively and interesting．

   One day he carefully explained a chemical reaction(反应) to the students of Grade10 inthe chemistry lab．

   “Be careful，everyone，”he said loudly，“Before I make the experiment，there are 25 atoms of carbon(碳原子)，but after I finish it，there are 24 atoms of carbon left!”

   He stopped to watch the classroom and hoped his students would go on explaining it．But the students looked at each other and nobody answered him．He had to ask，“What happens?What makes the atom lost?”

   The classroom was very quiet and none of the students looked at their teacher．

  “Who can tell us where it has gone?”

   Suddenly a soft voice came from the back row，“We did not see anybody leave the lab．”