Doublets

He published the rules in a book called Doublets.

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 word ladder puzzle between words "cold" and "warm" can be solved like this: COLD → CORD → CARD → WARD → WARM.

In this game you choose the number of steps, but if you are able to solve a puzzle in less steps, that\'s great!