Three missionaries, three cannibals, one boat that holds two
Three missionaries and three cannibals must cross a river in a boat that holds at most two people. If cannibals ever outnumber missionaries on either bank, the missionaries get eaten. The boat can't cross empty. Find a sequence of crossings that gets everyone across safely.
Reveal the answer
Send two cannibals across, then return one. Send the remaining two cannibals across, then return one. Send two missionaries across, then return one missionary and one cannibal. Send the last two missionaries across, then return the cannibal left behind. Finally send the last two cannibals across. Eleven single-person boat trips in total, and missionaries are never outnumbered on either bank. It's one of the oldest river-crossing puzzle families, adapted from the medieval 'jealous husbands' problem and later used by Saul Amarel as a founding example in AI search research.