Goes like this...
POST #1: " If gravity exists, then people don't float! XD"
POST #2: " If people don't float, then they have to walk! XD"
POST #3: " If people have to walk, then they invent bikes and cars! XD"
and etc...
basically, it's " IF, THEN" and the last THEN, becomes the next IF and it goes on like that! XD
(SO! The next person who posts will have to start their post with
" If people invent bikes and cars, then [fill this part in] ")