PolyGen is a program for generating random sentences according to a grammar definition, that is following custom syntactical and lexical rules. Formally, it is an interpreter of a language itself designed to define languages, where to interpret means executing a source program in real time and eventually outputting its result. Here, a source program is a grammar definition. The execution consists of the exploration of such grammar by selecting a random path, and the result is the sentence built on the way.