In this game, one poster would say "Let's pretend that ___" and the other person would pretend that it happened and said "Let's pretend___". It may not be the best explanation, so here's an example!
Poster 1:"Let's pretend that fish could walk on land with their fins and nobody knew."
Poster 2:"Hey, why is that fish walk on his fins?
Let's pretend that the sky was green instead of blue."
All SPPF rules apply.