A guard will drown you for lying — but what if you predict he'll drown you?
A traveller wants to cross a bridge guarded by a logician, who warns: 'If what you say next is true, I'll let you pass. If it's false, I'll throw you in the water.' The traveller replies: 'You will throw me in the water.' Can the guard either let him pass or throw him in without breaking his own rule?
Reveal the answer
No — both choices contradict the rule. If the guard throws him in, the statement was true, which should have earned passage. If he lets him pass, the statement was false, which should have meant drowning. Jean Buridan posed this in his 14th-century Sophismata as a hard case for medieval logic, showing how ordinary rules for truth and conditional promises can break down when a statement refers to its own outcome.