Word Ladder — free one-letter-at-a-time word puzzle

You are given a four-letter word and a four-letter target. Change one letter at a time until you reach the target, and every step in between has to be a real word. COLD becomes WARM in four moves: COLD, CORD, CARD, WARD, WARM. That is the whole game, and it is much harder than it sounds, because the letters you are most eager to fix are often the ones you have to break again.

Each ladder comes with a move budget: the shortest route plus three spare moves. Clear a ladder and the next one is dealt, longer than the last; run out of moves and the run is over. Your score is the number of ladders you climbed. Every ladder is generated by searching the word list for a route before it is dealt, so the route always exists and it can always be walked using ordinary words. Free, instant, nothing to install.

How to play

• Tap a letter of your word to pick the position you want to change, then tap a letter on the keyboard to change it.
• The new word must be a real word. If it is not, nothing is spent.
• Each accepted word costs one move. Stepping backwards costs a move too — there is no free undo.
• Reach the target and the next ladder is dealt with a fresh budget.
• The budget is the shortest possible route plus three. Ladders start three rungs long and stretch to five.
• On a keyboard, the arrow keys move the highlighted position and any letter key changes it.

Lewis Carroll invented this, and the maths says swap the vowel first

Lewis Carroll invented the puzzle on Christmas Day 1877 and called it Doublets. He ran a series of them in Vanity Fair from March 1879, with prizes, and the form has barely changed since: two words of the same length, one letter at a time, every rung a real word. COLD to WARM is the demonstration everyone still uses, because it contains the trap the whole puzzle is built on. The obvious first move is to fix the W, and the only four-letter word that lets you is WOLD, which almost nobody has met. The route runs COLD, CORD, CARD, WARD, WARM instead, which means moving two letters you thought were already right.

The useful habit is not obvious and it is worth stating as a number. Across the 1,409 everyday four-letter words this game deals from, changing the first letter leaves you an average of 2.67 legal words to choose from. Changing the second leaves 0.67. Sorted by what the letter is rather than where it sits, a consonant slot has 1.90 legal swaps behind it and a vowel slot has 0.69 — barely more than a third as many.

So the vowel is the bottleneck, and the practical rule follows straight from that: if the target needs a different vowel, make that move early, while you still have budget to walk around a dead end. The first letter is the widest door in the language — COLD alone opens onto BOLD, FOLD, GOLD, HOLD, MOLD, SOLD and TOLD — so it will still be there later. Fixing the easy letters first feels like progress and is usually how a ladder is lost.

The other habit is to climb from both ends. Look at the target and ask what could come immediately before it, then check whether that word is within reach of where you stand. Thinking costs nothing; only accepted words cost moves. And keep count: the budget is the shortest route plus three, which buys you exactly one wrong turn, the step back out of it, and one move to spare. If you are two moves from where you started with nothing gained, the ladder is probably gone, and the rest of the budget is better spent on your boldest idea than on careful shuffling. If you want a gentler word game, Word Scramble gives you the letters and asks for the anagram, and Word Wheel is nine letters with no target at all.

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