Word ladder
Word ladder (also known as Doublets, word-links, change-the-word puzzles, paragrams, laddergrams, or word golf) is a word game invented by Lewis Carroll. A word ladder puzzle begins with two words, and to solve the puzzle one must find a chain of other words to link the two, in which two adjacent words (that is, words in successive steps) differ by one letter. History. Lewis Carroll says that he invented the game on Christmas day in 1877. Carroll devised the word game for Julia and Ethel Arnold. The first mention of the game in Carroll's  diary was on March 12, 1878, which he originally called "Word-links", and described as a two-player game. Carroll published a series of word ladder puzzles and solutions, which he then called "Doublets", in the magazine "Vanity Fair", beginning with the March 29, 1879 issue. Later that year it was made into a book, published by Macmillan and Co. J. E. Surrick and L. M. Conant published a book Laddergrams of such puzzles in 1927. Vladimir Nabokov alluded to the game using the name "word golf" in the novel "Pale Fire", in which the narrator says 'some of my  records are: hate—love in three, lass—male in four, and live—dead in five (with "lend" in the middle).' The game was revived in Australia in the 1990s by "The Canberra Times" as "Stepword". Word ladders are often featured in the "New York Times" crossword puzzle. Rules. The player is given a "start word" and an "end word". In order to win the game, the player must change the start word into the end word progressively, creating an existing word at each step. Each step consists of a single letter substitution. For example, the following are the seven shortest  solutions to the word ladder puzzle between words "cold" and "warm", using words from Collins Scrabble Words. As each step changes only one letter, the number of steps must be at least the Hamming distance between the two words – four in the above example. Lewis Carroll's example has an extra fifth step as the third letter changes twice. Often word ladder puzzles are created where the "end word" has some kind of relationship with the "start word" (synonymous, antonymous, semantic...). This was also the way the game was originally devised by Lewis Carroll when it first appeared in "Vanity  Fair". Some variations also allow the player to add or remove letters, and to rearrange the same letters into a different order (an anagram). Five-letter word ladders. Donald Knuth used a computer to study word ladders of five-letter words. He believed that three-letter word ladders were too easy (although Lewis Carroll found six steps were required for APE to evolve into MAN), and that six-letter word ladders were less interesting, since relatively few pairs of six-letter words could be connected with a word ladder. Knuth used a fixed collection of 5,757 of the most common  English five-letter words, excluding proper nouns. He determined exactly when two words of the collection had a word ladder between them via other words in the collection. Knuth found that most words were connected to each other, and he also found that 671 words of the collection did not form a word ladder with any other words. He called these words "aloof", because "aloof" is itself an example of such a word.