Well in expository writing, you're intending to explain something. If you were writing about a banana and how you're suppose to peel it, what are the facts; how do you peel a banana. Is it true what you're trying to explain in your writing? If it is true are there facts to prove it, if so include them.
At the beginning of every story, the writer needs to prepare the reader of the events following. To accomplish this, the writer should highly rely on reality, especially physical reality, like biology.
