I believe that it's is correct only when used as a contraction, but not as a possessive. This is odd because 's is used to indicate possession in other words. I believe that's the rule. If you can replace it's with it is or with it has, then it is correct; otherwise, leave out the apostrophe.
This is exactly correct ! It is unique because every other word that shows possession always has an apostrophe just for that purpose. Every other contraction also has the apostrophe to show it is a contraction. Therefore, we only use the apostrophe when it is used as a contraction, and not to show possession when writing "its".
Now, at long last I total'y understand why my dad was sooo confus'd when I wrote him to tell him I had gott'n a contraction in Vietnam. I put the apostrophe in the wrong place when I said the doctor's told me "it's" ****" He thought that since it was simply writt'n it's or "IT IS" rather than the possessive form, that someone else had contract'd it and I was spill'g the beans about someone he didn't even know! Relief at last. Now, I've [got] another question. No I don't. Nice to see ya' around Pet'r!
Nice to be back, Bobby! As you and Yvonne already know, I've been very ill in the past three months or so but, at this point, I believe that I am scratching and clawing my way back to normalcy (for ME, anyway). Release the Kraken!!
English is full of these sort of exceptions that prove rules. Possessive "its" is one of them. Off the top of my head, another is the plural of "money", which is a slightly strange concept anyway. "Money" can be singular or plural, but if you use "monies", the usual rule is broken. Words ending with -y become -ies and words ending with -ey become -eys, except with "money". So you have donkeys and monkeys, but you have monies.