Help me understand this joke?

i was in science and we are preparing for our state testing. my teacher (a guy) is talking to the class when my language arts teacher (another guy) comes into the class and brings us dictionaries because tomorrow we are taking a writing test and we are allowed to use dictionaries. my language arts teacher walks over to my science teacher and my science teacher says jokingly (referring to the dictionaries) "what are those for? the expository or suppository essays or something like that?" then my language arts teacher says "the essays are supposed to flow, but not that way." then they both laughed. why is this funny/what does it mean? does it have something to do with the word suppository?