Everyone. An announcement. Apostrophes are only used when denoting possession. There is no apostrophe when you are talking about apostrophes. There IS an apostrophe when you are talking about apostrophes' purpose. It is THEIR puropose. their POSSESIVE purpose!!!!!! Ughhhhhhhhhhhhhhhhh.
If you're going to get on your grammatical high horse you want to make damn sure what you say is correct. Apostrophes are also used to denote contractions, such as 'can't, I'll' etc. Furthermore it is acceptable to use an apostrophe when you are pluralising an individual letter such as mind your p's and q's.
And like all good rules in english, there is an exception for your rule about denoting possession: its = belonging to it - it's = it is.