The farmer, the wolf, the goat and the cabbage
A farmer must cross a river with a wolf, a goat and a cabbage. His boat carries only himself plus one item at a time. Left alone together, the wolf will eat the goat, and the goat will eat the cabbage. How does he ferry all three across safely?
Reveal the answer
Take the goat across first (the wolf and cabbage are safe left together). Return alone, take the wolf across, but bring the goat back with you. Leave the goat, take the cabbage across. Return alone and fetch the goat. It's one of the oldest river-crossing puzzles on record, appearing in Alcuin of York's 'Propositiones ad Acuendos Juvenes,' a manuscript of puzzles compiled around 800 AD for Charlemagne's court.
— Alcuin of York, Propositiones ad Acuendos Juvenes — c. 800 AD