It's with an apostrophe is an abbreviation for it is, because you never, and I I think those three little rules there only apply only nouns have an apostrophe, pronouns never have an apostrophe and it's only nouns ending in s, have an apostrophe and it's only when they own the thing that follows, except when they, when they don't end with an s, but  you know the exception of men's, women's, children's and sheep.