So this guy walks ito a bar...
He sits down and orders a beer, and when he's finished it, he announces to all present that he can guess the vintage of anything he tastes, if someone buys him the drink. If he misses, he'll pay.
  A few patrons are just drunk enough to take his bet, and soon he's enjoying free shots and glasses of expensive wine, guessing flawlessly each time. Everyone is amazed.
Suddenly an unkempt, smelly old man down at the end of the bar staggers off his stool and joins the crowd, brandishing a bottle.
"Betcha can't guess this one, sonny!" he slurs, offering the bottle. "It's a little of my private stock..."
The guy is hesitant, but everyone eggs him on and he wants more free drinks, so he grabs the bottle and takes a swig, only to start spluttering and choking.
"Goddamn it, that's piss!" he shouts.
"Yes, it is", says the old man, grinning, "but... how old am I?"

LMAO!

That's a good one!