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River crossing

Wolf, goat and cabbage; missionaries and cannibals; soldiers and boys. Get everyone across without breaking a rule, in the fewest crossings.

  • Logic
  • Untimed
  • 8 levels

How it plays

Everyone starts on one bank and must reach the other. Each move picks who rows across; the banks, the boat and every crossing so far are listed above the moves. A bank must never break the puzzle's rule once the boat lands.

Par is the fewest crossings, found by trying every sequence breadth-first. Each level opens a harder puzzle, from two and two up to three missionaries and three cannibals. Choose whether a rule-breaking move is refused or ends the attempt.

Keys

Space starts and skips ahead · 1–9 pick · Enter submits · Esc stops. On a phone, tap.

What it trains

This practises state-space reasoning — seeing that the way forward sometimes means bringing something back. It is practice at these puzzles and claims nothing wider.

Learn

How to get better at river crossing: the method, the techniques, what each new level asks of you, and the traps the wrong answers are made of.

Your approach to each question

  1. Read which puzzle it is under the names. Fox, goose and beans is wolf, goat and cabbage; sheep and wolves, hikers and bears, guards and thieves are missionaries and cannibals; adults and children, knights and pages are soldiers and boys.
  2. Note which way you are going — left to right or right to left — and the par. Par is the fewest crossings unless extra crossings are allowed.
  3. Before each move, picture both banks after the boat lands, with the passengers counted on the bank they reach. Only tap a move when both banks are safe.
  4. Every crossing that goes over must be followed by one that comes back, so plan in pairs: who goes, and who brings the boat home.

Techniques and tricks

  • The goat goes first and comes back

    In the wolf, goat and cabbage family, the middle one clashes with both others. Take it over first; bring it back once, when you have just delivered one of the others; take it over last.

    e.g. Farmer + goat →, ← farmer alone, farmer + wolf →, ← farmer + goat, farmer + cabbage →, ← farmer alone, farmer + goat →: 7 crossings.

  • Four crossings per soldier

    In the soldiers and boys family, the boat takes one soldier or two boys. Each soldier costs four crossings — both boys over, one back, the soldier over, the other boy back — and one last trip takes the two boys.

    e.g. One soldier takes 5 crossings, two take 9, three take 13.

  • An empty bank of the few is safe

    In the outnumbering family, a bank with no missionaries at all can hold any number of cannibals. So park cannibals on the far bank early: a bank of only cannibals is safe while the missionaries wait.

  • Bring back a pair in the middle

    The hardest moment in three missionaries and three cannibals is getting the last missionaries over. The fix that feels like going backwards is to bring one of each back together, so both banks stay level.

    e.g. Three and three in a boat of two takes 11 crossings, with “← 1 missionary + 1 cannibal” as the sixth.

  • Never undo the last crossing

    Sending back exactly who just came over returns you to where you were, two crossings poorer. If the only safe move is an undo, the mistake was the move before it.

  • Solve it from both ends

    The rules are the same on both banks, so a solution run backwards is still a solution, and the last crossings can mirror the first. If you know how it starts, you know how it can end; only the middle needs thought.

As the levels climb

  • Each level opens one more puzzle, in a fixed order: two and two, one soldier, wolf, goat and cabbage, two soldiers, three soldiers, four and four with a boat of three, five and five with a boat of three, and last the classic three and three with a boat of two.
  • Half the time you get the newest puzzle; otherwise any you have already opened. Keep the earlier ones sharp — their fewest crossings are fixed, and you should hit them every time.
  • The soldier puzzles get longer but not harder: repeat the four-crossing cycle per soldier. The crowd puzzles change shape: four and four and five and five use a boat of three, and the classic three and three, with a boat of two, comes last. Each needs its own middle move that keeps both banks level.
  • Untick a family in “Puzzles” and its rungs drop out of the ladder, so it ends sooner.

Traps to avoid

  • Forgetting the bank you leave: in wolf, goat and cabbage, taking the wolf first leaves the goat with the cabbage. Check what stays behind before checking what arrives.
  • Counting the boat as neutral: the passengers join the bank they land on. Two cannibals landing beside one missionary outnumber him, even if they stay in the boat.
  • Tapping and hoping: with “Is refused, and counts”, a refused move still adds one to your count, so it costs you against par. With “Ends the attempt”, it ends the puzzle.
  • Sending a soldier back: it costs two crossings for nothing. In that family, only boys row back.

How to practise

  • Tick one family at a time in “Puzzles” until you can do each of its rungs at par, then mix them.
  • Raise “Extra crossings allowed” to 2 or 3 while you explore a new puzzle, then lower it to 0 so only the fewest counts.
  • Set “A move that breaks a rule” to “Ends the attempt” once you know the puzzles, so every move has to be checked in your head first.

Levels

It is untimed unless you turn the clock on. There are 8 levels with the default settings, and the counter stops at the real top. Your level is judged on your last ten answers, every ten: 90% moves you up, below 80% moves you down, which holds most people near four in five right.

All of it is in the settings beside Start — how many answers a judgement looks at and how often it recalibrates, the thresholds, the time, which questions, how many options, and whether you see each verdict at once or review them together at the judgement. You can also jump to any level.

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