Drunk

One day, a father and his little son were going home. At this age, the boy was interested in all kinds of things and was always asking questions. Now, he asked, "What's the meaning of the word 'Drunk', dad?" "Well, my son," his father replied, "look, there are standing two policemen. If I regard the two policemen as four then I am drunk." 

"But, dad," the boy said, " there's only ONE policeman!"