This sentence is false — is it true?
Consider a single sentence: 'This sentence is false.' If it's true, then what it says must hold, meaning it really is false. But if it's false, then its own claim to be false is wrong, which makes it true. Try to assign it a consistent truth value.
Reveal the answer
You can't — every assignment flips into its opposite. Logicians trace the puzzle back to the 6th-century-BCE Cretan Epimenides, who reportedly said 'all Cretans are liars', though that version has an escape hatch: it can simply be false without contradiction, since not every Cretan need be a liar. The sharper, inescapable form used here is generally credited to the ancient philosopher Eubulides of Miletus, and it's still cited by logicians such as Alfred Tarski as proof that no language can fully contain a consistent theory of its own truth.
— Attributed to Eubulides of Miletus, Liar paradox — 4th century BCE