A classic of delicate maneuvers: moving forward does not always mean getting closer to the solution, and each movement has consequences on both sides at the same time.
Missionaries and cannibals
Riddle statement
Three missionaries and three cannibals must cross a river in a boat that only accepts one or two people.
On no shore can there be a group in which the cannibals outnumber the missionaries, unless there are no missionaries on that shore.
Is it possible for them all to cross safely? If yes, in how many minimum journeys?