One night a man decides to visit his local bar. He takes a seat and orders a beer. After polishing off his beer, he beckons the bartender over and says, "Betcha $20 I can bite my eye."
The bartender scoffs and accepts. The man then calmly removes his false eye and bites it. The bartender grudgingly forks over a twenty.

Later that night, after a few more beers, the man wanders back to the bar and says rather drunkenly, "Hey barkeep, betcha another $20 I can bite my other eye."

Wanting to win back his money and seriously doubtful that the man has two false eyes, the bartender accepts. The man calmly removes his false teeth and bites his other eye. Scowling, the bartender hands over another twenty. The man leaves and wanders around the bar as he drinks a few more beers.

He strolls back over to the bar, leaning on it, again and calls the bartender, "Hey, barkeep," he burbles, "I'll give you a chance to win yer money back plus. Betcha $100 if you put a shot glass on that end of the bar, and I stood on this end, I could piss into it and not spill a drop."

The bartender eagerly accepts, knowing the feat to be impossible. The man wobbily climbs atop the bar, zips down his fly and promptly pisses all over the bar. He zips up, sits down, slaps the $100 on the bar and laughs uproarously.

"What's so funny?" says the barkeep. "You just lost everything you won and more!"

Well," giggles the man, "I just bet those guys over there $200 that I could piss all over your bar and you wouldn't get angry."