The game is
simplestupid: you come up with a world that is different from ours in one (relatively major) way. You ask a question about that world (which doesn't have much to do with the world itself). Then the other person comes up with a ridiculous answer to the question, and comes up with a world and asks a question of their own. You can justify your answer, just make it equally nonsensical.
Person 1: In a world where pencils are the primary source of food, what are lamps made from?
Person 2: Dead elephants. In a world where everyone is forced to wear pink, where do cats typically live?
Person 3: Uganda.
Person 3 would then think of a world and a question.
To start: In a world where the consumption of popcorn is a sin, why are video games banned?