An old guy walks into a bar and asks for a bottle of forty-year old Scotch.  The bartender, not wanting to give up the good liquor, pours a shot of ten-year Scotch and figures that the guy won't be able to tell the difference.  The guy downs the Scotch and says: "This Scotch is only ten years old!  I specifically asked for forty-year old Scotch."
Amazed, the bartender reaches into a locked cabinet underneath the bar and pulls out a bottle of twenty-year old Scotch and pours the man a shot.  The guy drinks it down and says, "That was twenty-year old Scotch.  I asked for forty-year old Scotch."
So the bartender goes into the back room and brings out a bottle of thirty-year old Scotch and pours the guy a drink.   By now a small crowd has gathered around the man and is watching anxiously as he downs the latest drink.  Once again the guy states the true age of the Scotch and repeats his original request for forty-year old Scotch.
The bartender can hold off no longer and disappears into the cellar to get a bottle of prime forty-year old Scotch. Soon, the bartender returns with the bottle and pours a shot. The guy downs the Scotch and says, "Now this is forty-year old Scotch!"  The crowdapplauds his discriminating palate.
An old drunk who had been watching the proceedings with interest, raises a full shot glass of his own and says, "Here, take a swig of this."
The guy takes the glass and downs the drink in one swallow. Immediately, he chokes and spits out the liquid on the barroom floor. "My God!  That tastes like piss," he yells.
"Great guess," says the drunk.  "Now, how old am I?"