Pretty simple, person 1 describes something they want, person 2 explains why they can't have it.
Example:
I want a turtle!
Example:
You get the idea. So to start us off,Person 1: I want a nuclear bomb!
Person 2: You can't have a nuclear bomb because it could cause mass devastation and make your mother very angry!
I want a pot of soup
Person 3: You can't have a pot of soup because this is MY pot of soup.
I want a boat
etc.
I want a turtle!