A dignified old gent walks into the bar and sits down at the bar.
"My good man, I would like a shot of 20 year old Scotch!"
The bartender figures the old guy can't tell the difference and pours him some 5 year old Scotch.
The old guy sips, and spits it out!
"Sir, this is only 5 years old, I distinctly ordered 20 year old!"
The bartender apologizes and digs around behind the bar and finds a bottle of 10 year old Scotch and pours a shot.
The old guy sips it again, and spits it out again.
"Sir! This 10 year old Scotch as anyone can tell! If you do not have 20 year old Scotch, I shall take my custom elsewhere!"
He apologizes again, and looks in the owner's office, and sure enough, in the guy's desk there's a bottle of 20 year old.
He pours the shot, the old guys sips it and sighs with pleasure.
"ah, that's the stuff my good man, another please!"
Meanwhile the guy who's sitting next to the old geezer pushes a glass in front of him.
"Try some of thish," he slurs drunkenly.
The old guy sips and spits it out instantly
"Good god man, that's not Scotch, it's PISS!"
"yeah, but can you tell me how old I am?"