20 Pints Of Guinness In 30 Minutes

I found myself in a pub in Cork in the south of Ireland. A group of American tourists came in.

One of the Americans said, in a loud voice, "I hear you Irish think you're great drinkers. I bet $5,000 that no one here can drink 20 pints of Guinness in 30 minutes."
The bar was silent, but the American noticed one Irishman leaving.
No one took up the bet.
40 minutes later, the Irishman who left returned and said, "Hey Yank, is yer bet still on?"
"Sure", said the American, "20 pints in 30 minutes for a bet of $5,000."
"Grand", replied the Irishman, "so pour the pints and start the clock."
It was very close but the last drop was consumed with 2 seconds to spare. "OK Yank, pay up", said the Irishman.
"I'm happy to pay, here's your money", said the American. "But tell me, when I first offered the wager, I saw you leave. Where did you go?"
"Well sir", replied the Irishman, "$5,000 is a lot of money to a man like me, so I went to the pub across the road to see if I could do it".