A man walks into a bar and says to the bartender, "I bet you fifty dollars that I can bite my right eye."

The bartender says, "Yeah, right! I’ve never seen anyone do that!" So the man takes out his glass eye and bites it.
The angry bartender pays the man his fifty dollars and the man walks away. He comes back half an hour later and says, "I bet you fifty dollars I can bite my left eye."

Now the bartender becomes really skeptical. She says, "I just saw you walk in here — you can’t be blind!"

So he takes out his fake teeth and bites his left eye. The bartender pays him his money and he walks away.