At this point in the book, you might have realized that, practical as it may be, game design happens in the designer's head first, on a document of some sort second, and only in a third step is it possible to implement it.
To actually make the game happen, the designer has to get their hands on a tool that allows them to create working software; to write some code. Not all game designers are programmers, and even if they know how to program, designing and programming are two different jobs both requiring a person's full attention (and the bigger the project, the more this truth cannot be bent).
There is one activity where a game designer is required to turn the design into a playable thing as if he was the only person working on it. This is the quintessence of practical game design.
This is the creation of a game prototype.
