THE MAN WHO SPOKE THE TRUTH BUT DIDN'T
A traveller was walking one day when he met an old man sitting beside the road smoking a pipe.
"The first thing said to you by the first person you meet today will not be true," said the old man. "Trust me - don't believe what he says!"
"OK," said the traveller. "But hang on a minute: you're the first person I've met today."
"Exactly!" said the old man.
You may have spotted something funny going on here. If the old man speaks the truth, then the first thing he says is not true. But if the first thing he says is not true, then the first thing he says is true.
The traveller thought he saw a way out of the paradox: claim that what the old man first said is neither true nor not true. After all, why does every such sentence have to be either true or not true.
"Old man, you're trying to trick me," said the traveller. "It's obvious that what you said is neither true nor not true."
"Aha," said the old man. "You're suggesting that it is not true that what I said is true, and also not true what I said is not true?"
"That's exactly right," said the traveller.
"Well, then, if it's not true that was I said is true, then what I said is not true!"
The traveller was starting to get a headache. The old man continued: "And if it's not true that what I said is not true, then what I said is true! For what I said is precisely that what I said is not true!"
The traveller was starting to feel like ramming the old man's pipe down his throat.
"So you see," said the old man, "your suggestion is wrong: it's not true that what I said is neither true nor not true. In fact, it's both true and not true!"
That one is kinda long, now ain't it.