Do you know how, in a lot of RPG's, there is an attribute system, where certain types of attacks are strong or weak to certain types of enemies? For example, how, in Pokemon, there are electric type pokemon who are strong against water type ones who are strong against fire type ones who are weak against rock type ones and so on, or how in Bakugan there's light which goes against darkness which goes against water and so on?
That is how this game will work; à la the stonecutter parable, every post will invent an attribute that is strong against the one someone posted before it. You also must explain why it's strong against it.
Example: Poster 1: Caterpillar Poster 2: Bird Poster 3: Windstorm Poster 4: Boulder
First person gets to choose the first attribute.