I kinda run by the rule that you put an "e" after "o" for plurals (like in tomatoes and potatoes) but "does" means lots of female deers or the singular 3rd person verb-form of "do." So you dump the "e" and put in an apostrophe to hold its place.
Of course, if English were a purely phonetic written language, all this would be moot.
Have you ever notice how much "apostrophe" resembles "catastrophe"? And it often is...
Knot-the-warped-literature-major-child-of-a-long-line-of-librarians