"Where is the supermarket?" a man asks Peter.

   "The supermarket? You have to cross abridge and then turn right."

   "And is the bridge long?"

   "Not very long. Thirty metres."

   The man says "Thank you!" to him and then goes towards thebridge.

   Suddenly the man hears someone shouting something.

   "Stop!"says Peter, "I'm sorry, sir. I made a mistake (错误) just now. You knowthe bridge is forty metres long. If you go thirty metres and then turn right,you'll fall into the river!"