(It's an oldie, but a goodie.)

There was this guy who went into a bar.  He went up to the bartender
and said, "Bartender, are you a bettin' man?"

The bartender replied, "Certainly!  I'm ALWAYS a bettin' man!"  To
which the man said, "I'll bet you $50 that I can lick my right eye."

The bartender thought about this a while and finally agreed to the bet.
The man reached up and pulled out his glass right eye and licked it.
The bartender groaned and begrudgingly gave the man his $50 telling him
to leave his bar.

A week or so later, the same man appeared in the bar.  He went up to
the bartender and said, "Bartender, are you still a bettin' man?"  The
bartender replied, "Certainly!  I told you I'm ALWAYS a bettin' man!"
To which the man said, "I'll bet you $100 that I can bite my left eye."

Well, the bartender thought he had him on this one!  There was no way
that he had TWO glass eyes so the bartender agreed.  The man reached up
to his mouth, pulled out his dentures and clicked them on his left eye.
The bartender moaned and paid the man his $100 telling him to get out
of his bar.

A week or so later, the same man ventured into the bar again.  He went
up to the bartender and said, "Bartender, are you still a bettin' man?"
The bartender said, although with a little caution this time,
"Certainly!  I told you I'm ALWAYS a bettin' man!"  To which the man
said, "Give me a shot of whiskey."  The bartender poured the man a shot
and he drank it down.  Slamming the glass on the bar he said, "I'll bet
you $500 that you can spin me around on this bar stool and I can piss
in that glass right where it lays and not miss a drop."

Well, the bartender's eyes lit up.  Here was one time that he was
certain that he would win!  "Agreed!"  he cried.  Coming out from
around the bar, he grabbed onto the man's bar stool and spun it as hard
as he could.

Well, the man just let loose and piss flew EVERYPLACE!  Not so much as
one drop even came close to the glass and the bartender was soaked.
When he was done, the bartender was laughing and laughing and holding
out his hand.  The man pulled out his wallet and gave him his $500.  But
the bartender was puzzled and as he was wiping off his face, he asked
the man, "Why did you bet me $500 that you could piss in that shot
glass on the bar when you had to have known there wasn't any possible
way to do it??"

The man just smiled and told him, "You may have won $500 off me but I
bet that guy over in the corner $10,000 that I could piss all over you
and your bar and you would just laugh!"