A one-passenger boat must ferry a wolf, a goat, and a cabbage across a river
You must get a wolf, a goat, and a cabbage across a river using a boat that carries you and only one of the three at a time. Left alone together, the wolf will eat the goat, and the goat will eat the cabbage. How do you get all three across safely?
Reveal the answer
Take the goat across first and leave it, then return alone. Bring the wolf across, but take the goat back with you to leave the wolf alone on the far bank. Leave the goat behind, bring the cabbage across to join the wolf, then return alone one final time for the goat. The puzzle appears as problem 18 in Propositiones ad Acuendos Juvenes ('Problems to Sharpen the Young'), a collection traditionally attributed to the 8th-century scholar Alcuin of York, making it one of the oldest documented river-crossing puzzles in Europe.
— Attributed to Alcuin of York, Propositiones ad Acuendos Juvenes — Problem 18, c. 9th century