To Apostrophe Or Not To Apostrophe

I have just read The Uncommon Reader by Alan Bennett. It's called a novella; however, it is more like a long short story and I wanted to write about it today. My schedule will not allow me to do it justice; so, I will defer it to another day. I do have a question, however. There was a small incident this morning which reminded me of one of those pearls of wisdom my mother always seemed to have at the ready (along with, "Anyone can say anything they want to about you; but, only you can make it be true," or "Don't believe anything you hear and only half of what you see," and "Don't put anything smaller than your elbow in your ear.") She told me that, "Before you criticize, make absolutely sure that you are absolutely right." Those words came flooding back to me this morning when I opened one of my many e-mails. It was from a young lawyer who is personal counsel to one of the corporations our firm is currently defending in a civil action. He wrote to say that he received the Answer we had prepared and filed with the Court for his client. It behooved him, however, to point out a grammatical error in the document. An error which, by the way, was a "pet peeve" of his. "You repeatedly wrote 'its' rather than 'it's' when writing in the possessive case, e.g. 'Its place of business is in New Jersey.'" Hmmm. Little did he know that he was communicating with a woman who is a storehouse of worthless information. Since another trait of mine is that I am simply awful at small talk, I have made a game of trying to work these trifling tidbits into cocktail conversation - with some modest success. The enterprise also keeps these unimportant factoids fresh in my mind. As it happens, I remember sitting in English class in the fourth grade. We had spent part of the class diagramming sentences. (I do not believe diagramming is taught much, if at all, anymore. A shame, really. I thought it was fun - like working out a puzzle.) We moved on to pronouns and the possessive case. "A thing cannot possess," Sr. Vera said to explain why an apostrophe is not used with the possessive, its. I have no idea why I remembered it all these years. Perhaps the rules of English grammar have changed since I was a child - which, I admit, was very close to the time Christ was one as well. So, the question is, am I still correct in believing the rule holds true? I'd like to be "absolutely sure" that I am "absolutely right" before I send any zingers into the great beyond (although my fingers are itching to).