So you make up a story from the point where the last poster left of.
For example:
Poster 1: there once was a girl who had a bread basket
Poster 2: and she was walking through the woods
I'll start.
There once was a little boy
For example:
Poster 1: there once was a girl who had a bread basket
Poster 2: and she was walking through the woods
I'll start.
There once was a little boy