(my best) Oldie but Goodie
A man stepped up to a bartender and said, "I'll bet you five bucks I can bite my eye."
The bartender thought about this a bit and said, "You're on," to which the newcomer took out his glass eye, put it in his mouth, and bit it. "Oh, you got me," said the barkeep"I'll bet you double or nothin' I can bite my other eye."
The bartender reflected on this awhile. Seeing as how the guy could only have the one glass eye and still see, he agreed. The patron took out his false teeth, raised them up, and bit his other eye. "Well, I'll be damned if you didn't take me again," chuckled the owner.
As he was reaching into the till, the customer said, "Hold on! I'll go you one further! I'll bet you double or nothin' I can git up here and piss across this bar and land every last drop in a shot glass." The bartender considered this but an instant before accepting this bet. The guy then climbed up, stood on a barstool, and proceeded to pee, getting it everywhere, on bottles, the bar, the bartender, managing to land nary a drop in the shot glass. The bartender was beside himself, guffawing and wiping the tears rolling down his cheeks. The man climbed down, took out a twenty, and handed it across.
"Thank you, kindly!" The bartender managed weakly. "But just what on earth gave you the fool impression you could pull that stunt off?"
Pointing, "Well, you see them fellers over there? I just bet them each $50 that I could git up on a barstool, piss all over you and your bar, and you'd laugh."