Mr. Black was very forgetful. Each time when Mrs. Black wanted her husband to do something, she had to tell him again and again not to forget it. Once Mr. Black was asked to post a letter, " Do remember to put it into the letter box on your way to work." The wife said. " This time I won't forget." Mr. Black said to his wife. "I will hold the letter in my pocket." "Oh, my dear, you may go now. Please come back early." Mrs. Black said. As Mr. Black was shown out, he felt as if his wife touched him affectionately   on the back. Hand in his pocket, Mr. Black walked slowly down the street. Soon a man caught up with him from behind and smiled at him, "Don't forget to post the letter." Mr. Black was wondering how the man knew he had a letter to post. Then a girl passed him and turned to smile at him, too."Now be sure to post the letter," the girl said. Mr. Black was even more puzzled, "Why are they smiling at me? And how do they know I'm going to post a letter?" he thought to himself. According to the passage we can infer Mrs. Black touched her husband on the back so as to  _   .
stick something on his back show her love warn him not to forget to post the letter let him go to work right now