The rules are simple. Somebody types three words, and whoever posts a reply will continue the story with three more words.
For instance, if a guy types "The guy was...", then the next reply might be something like "...running down the..." and so on.

Special "words" like "you're" or "I'm" that are actually two words will still be classed as one word.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram