We basically write a neverending story. You write 3 words per post. You can use punctuation partway through a post as well.
Example:
Poster A: Once upon a
Poster B: time, there was
Poster C: a cat. He
I’ll start: In a forest
full of trampolines,
there was a
cat named Randy.
Randy was mean
He threw a
hissy fit because
he hated a
dog named Kirby.
He ate schoolbuses
filled with chocolate.
Unfortunately, the schoolbuses
smelled like chocolate.
The people inside
ate the chocolate
before Kirby could.
Kirby was angry
so he barked
and had indigestion.
He was shaved