Word ladder
A word ladder is a puzzle in which you transform one word into another by changing a single letter at each step, where every intermediate step must itself be a real word of the same length.
Lewis Carroll invented the form in 1877 and called it Doublets. COLD becomes WARM in four moves: COLD, CORD, CARD, WARD, WARM. Each rung differs from the one below it by exactly one letter, and no rung may be a non-word, which is the constraint that makes the puzzle hard.
The difficulty is rarely where it looks. Some pairs have a short chain that is easy to miss, some have only a long winding one, and some pairs have no chain at all because one of the words is isolated — no real word sits a single letter away from it. Computer scientists use word ladders as a teaching example for shortest-path search, since the words are points and the one-letter changes are the edges between them.
Common variants allow adding or removing a letter as well as swapping one, which connects words of different lengths and makes far more pairs reachable.
Daily Anagram Puzzle — a related climb — each rung rearranges a whole word rather than swapping one letter.