Puzzles
Three couples, one boat, and a very jealous constraint
Three married couples need to cross a river in a boat that holds at most two people at a time. No woman may ever be in the company of a man who isn't her husband — on either bank or in the boat — unless her own husband is also present. How do all six get across?
Reveal the answer
It takes eleven one-way crossings, using trips where pairs of women cross together and a husband-wife pair ferries back to keep the rule intact throughout. The puzzle traces back to Alcuin of York's Propositiones ad Acuendos Juvenes (c. 800 AD), originally posed with siblings rather than spouses; Niccolò Tartaglia extended it to four couples in 1556 and proved that version is unsolvable without a mid-river island.