A Drink

This guy walks into a bar and orders a drink. The bartender gives him the drink and he drinks it. Then he looks in his shirt pocket and orders another drink. He does the same thing with the second drink and orders a third drink. The bartender pours the drink and says "listen pal I'll buy you drinks all night long if you tell me why you keep looking in your shirt pocket". To this the guy replies "I have a picture of my wife in my pocket and when she starts to look good I know I've had enough to drink and it's time to go home".