River Cross

Six crates are waiting on the near bank, labelled A to F and drawn as six different shapes. The ferry holds two of them, and it may cross empty. The catch is on the notice board: certain pairs of crates must never be left standing together on a bank while the ferry is on the other side. Leave one of those pairs alone and the round is over on the spot.

The notice is published, not hidden. Every forbidden pair is printed above the board before you touch anything, and the board tells you which pair the load you have chosen right now would strand. Nothing is a surprise, so this is a planning puzzle rather than a guessing one — and the plan almost always involves carrying something back, which is the whole lesson.

Every board is searched before it is dealt. A breadth-first search walks all 128 positions those six crates can be in, following only the crossings that leave nobody in danger, and the board is only dealt if the search actually reached "all six across". The crossings you are given are that shortest route plus four spare, and the number still needed is printed while you play.

It runs entirely on your device: 0 network requests, works offline. No ads, no account, nothing to buy, and no betting, money or prizes of any kind.

1. Focus the board — click it, or press Tab until it is focused.
2. Read the notice first. The pairs listed there are the only rule in the game. Each crate is named by a letter and drawn as a shape, so a pair reads the same in colour, in greyscale and out loud.
3. Choose a crate with the left and right arrows. The cursor walks all six, wherever they are standing.
4. Press Enter to put the chosen crate on the ferry, and Enter again to take it back off. The ferry holds two. Loading and unloading are free — only crossing costs anything.
5. Press the up or down arrow to cast off. The ferry only ever has one bank to go to, so either key crosses. This is the one move that spends a crossing, and it works even with an empty ferry — coming back for something is a normal part of the answer.
6. Watch the warning line. Before you cast off it names the pair that would be left alone on the bank behind you, if there is one. If it is empty, the crossing is safe.
7. Press Esc to take everything back off the ferry. Changing your mind never costs a crossing.
8. Watch "shortest from here". It is the fewest crossings that still finish the board from exactly where you are standing. If it goes up, the last crossing cost you; when it reaches zero you have won.

You can play entirely by pointer instead: click a crate to load or unload it, and click the ferry to cast off.

How do you get everything across a river when some pairs cannot be left alone?

You stop thinking of it as moving crates forwards and start thinking of it as choosing, at every crossing, which crates are safe to leave behind. Work out which crate is in the most published pairs — that one usually cannot be left with anybody, so it has to travel back and forth with you rather than sit on a bank. Then the route writes itself: take that crate across with a partner, bring it back alone, swap the partner, and repeat. The move that feels like a mistake, ferrying something back the way it came, is almost always the move that makes the next one legal. On these boards the shortest answer is seven or nine crossings, and every one of them is found the same way.

Why does bringing a crate back ever help?

Because progress here is not the number of crates on the far bank — it is which crates are standing together without supervision. A bank with four crates on it can break a rule that the same bank with three does not, so removing a crate from a bank can be worth more than adding one to the far side. That is the single idea this puzzle exists to teach, and it is why counting how many you have got across is a bad way to tell whether you are winning. The "shortest from here" number is the honest one: it is a fresh search from your actual position, so it goes down only when the crossing you made genuinely helped.

Is every board actually solvable?

Always, and it is proven rather than assumed. Before a board is dealt, the search walks every one of the 128 positions — which crates are across, and which bank the ferry is at — following only crossings that leave no published pair alone. A board is dealt only if that search reached the goal, and the crossings you are given are the depth it found plus four. Most randomly drawn notices are thrown away for exactly this reason: with six crates, more forbidden pairs than the ferry can separate makes the far bank unreachable, and those boards never appear.

What happens when I leave a forbidden pair alone?

The round ends immediately and the pair is named. That is deliberate: a board in that position has already been lost, and letting you keep ferrying would only postpone the same answer. The same is true when the shortest remaining route no longer fits the crossings you have left — the round ends there and tells you how far short it was. Four spare crossings is room for two wasted round trips, so losing usually means one crossing made in the wrong order rather than a long slide.

Is there any money, betting or in-app purchase in this?

None, and there never will be. No currency, nothing to buy, no hints to unlock for a fee, no prizes.

Does it work offline?

Yes. Once the page has loaded once, the whole game — the board generator, the search that proves the far bank reachable, and the one that counts your shortest remaining route — is on your device. No server is involved in play at any point.

Can I play it with a keyboard or a screen reader?

Both. Arrows choose a crate, Enter loads and unloads it, up or down casts off, and Esc empties the ferry. Every crate announces its letter, its shape and which bank it is standing on; the notice is read out as a list of letter pairs; and the warning names the pair a crossing would strand before you make it. Nothing on the board is carried by colour alone — a crate on the ferry says so in words, and the shapes distinguish the six even in greyscale.

Six crates, always, and a ferry that holds two. The notice lists between four and seven pairs, drawn so that the board is solvable in seven or nine crossings — five is what an unconstrained board costs, so anything that easy is thrown away, and nine is the deepest six crates can be. The budget is always the shortest route plus four. Every board takes an odd number of crossings, which is not a quirk: the ferry starts on the near bank and has to finish on the far one. There is no timer, no hint button and no undo of a crossing — the warning line before you cast off is the whole safety net, and it is deliberately shown rather than buried.

Water Jugs is the other puzzle here whose boards are proven by a search before they are dealt, and Rope Bridge is the other one about who travels back. Route Plan and Toll Roads are the shortest-path puzzles, and Gap Logic is the deduction puzzle where, as here, the constraints are printed rather than discovered.

Bookmark this page (Ctrl+D, or ⌘D on Mac) or install the app — it works offline the next time you need it.