John goes to a pub and has a quiet drink at the bar. Next to him sits another guy, Jim. They start chatting a bit, and the conversation turns competitive. Jim says "Hey, John, I bet you a drink that I can bite my eye". John thinks "that's impossible", and tells Jim "OK, you're on". Jim takes out an artificial eye and bites it. John feels played, but buys him a drink.
A while later, Jim says to John "hey, I bet you a drink I can bite my eye". John says, "Yeah, I fell for it once, I know". Jim: "No, I mean my other eye". John's thinking, no way, he can't have both artificial eyes, he sees well enough after all. He hays "OK, Jim, you're on". Jim pulls out his dentures and bites the other eye. John's shocked, but pays his debt.
A bit later, Jim says "Hey, John, I bet you a drink that I can fuck your wife and her pussy will turn blue." John is perplexed, that ain't fucking possible! How many times he roughed it out with her, and he knows that a pussy never turns blue... "OK, Jim, you're on." They go to John's place, John explains to his wife, her and Jim do the deeds while John watches. Minutes after minutes, time passes, Jim and John's wife go on and on and on... At some point Jim say, "I'm done, John". Ecstatic, John exclaims "AHA, YOU SEE, IT DIDN'T TURN BLUE!", pointing to his wife's pussy. Jim looks back and says "You're right, John, I owe you a drink."