A father enjoys inviting his four-year-old son to take a sip when he drinks.One day,the father drives his son to eat out.In the resturant,the father asks his son to take a sip again.On the way home,the policeman detecs him with the alcohol detector.Unfortunately,there is excessive drinking.The father says:' I dont think this detector is exact,because I failed to drink at all.Then he asks his son to be deteced,and the result is the same.The father says to the policeman:see,how could my four-year-old son drink?The father quickly drives away,when the policeman feels quite puzzled.