He stands up and says "If anyone here can drink 10 pints of guiness in 10 minutes I'll give him 100 dollars!"
No one answers him and one man walks out of the pub.
The American goes back to his drink and someone taps him on the shoulder 15 minutes later. It was the Irish man who had walked out earlier. "Does your bet still stand?" He asks.
The American says yes and gets him his pints. The Irishman then drinks all 10 in 10 minutes.
The American gives him his money, he says "That was amazing! But why did you leave earlier?"
The Irish man says "I went to the pub across the road to make sure I could do it first!"