If are familiar with liars paradox this puzzle will probably be no challenge for you. There is a lot of riddles on the same basis.
Trail
You are sentenced at court.
Judge is a fair guy so he offers you following:
“You can say one sentence to defend yourself. If you lie we will hang you. If you tell us the truth we will shoot you.”
What will you say?
Hint
Say something so that the judge can’t keep his word.
Answer
“It’s certain that you will hang me”
If the judge decides you were right he should shoot you. But you would have lied in that case.
And if he decides you lied he should hang you but it would mean you told the truth.
Friend told me about another solution (probably easier but not that nice :))
If you just say: “I’m lying right now” the judge probably won’t be able to solve it either...