"A Scotch Expert" joke

A man walked into a bar and ordered a twelve-year old scotch. As the bartender was busy, he decided to serve the guy whatever he had under his hand.
The fellow took a sip, spat it out, and told the bartender, "I asked for a twelve-year old scotch and not a three year-old one."
When the bartender heard that, he checked the bottle and was amazed that the fellow was right -- he had served him a three-year old scotch. The bartender wanted to see how good the fellow was, so he served him another scotch, this one a six-year old.
The guy had a sip and spat it out, complaining, "I asked for a twelve-year old scotch and not a six-year old one."
The bartender was surprised by how good this fellow was. So the barman decided to play one more little game. He served the customer a nine-year-old scotch instead of a twelve-year old one as requested.
The fellow had a sip and spat it out, stating, "I think I asked for a twelve-year old scotch and not a nine-year old one."
The bartender was very impressed and finally served him the demanded twelve-year-old scotch.
The customer took a sip and added, "This is what I asked in the first place."
At the end of the counter sat a man who had witnessed this scene. He sent a tumbler to the scotch expert and asked him to have a sip.
The fellow did so and spat it out and said, "Good Lord, that's piss."

The other man added, "Now tell me how old I am."