Tell the truth and you may drown. Lie and you may drown. What do you say?
A traveler wants to cross a bridge guarded by an armed man, who warns: "Swear an oath. If it's true, you may pass; if it's false, I'll throw you into the water." The traveler swears: "You will throw me into the water." If the guard throws him in, the oath was true, so he should have been let across. If the guard lets him pass, the oath was false, so he should have been thrown in. What should the guard do?
Reveal the answer
Medieval logician Jean Buridan posed this in his 14th-century "Sophismata" as a bridge-crossing cousin of the liar paradox: any outcome the guard picks contradicts his own rule. Buridan used it to probe the limits of truth-conditional reasoning when a statement's truth depends on an action taken in response to the statement itself, a self-referential snare that still unsettles formal logic today.