A crocodile steals a child, and will return him only if the parent predicts correctly

The crocodile tells the parent: guess correctly whether I'll return your child, and I will. The parent replies, 'You will not return him.' Now the crocodile is stuck — returning the child proves the parent wrong, breaking its promise; keeping the child proves the parent right, which should also mean returning him. Attributed to the Stoic philosopher Chrysippus, it's one of the oldest self-referential paradoxes on record.

— Attributed to Chrysippus, Crocodile dilemma — 3rd century BCE