We guess that more red ink finds its way to "its" than to any other word, and yet the problem persists. It's not that our pupils want to be difficult - they just don't understand the system. Why, exactly, did we write "its" (twice) in the first sentence but "it's" in the second? Here's an example of grammar where the conventions do make sense, so we may as well help young writers to see the deeper reasons behind what can otherwise look like arbitrary rules to be memorised.
The easy one is "it's". This is the ordinary apostrophe of omission, where 's stands for "is" or "has". If you can expand it to "it is" or "it has", then you need the apostrophe. In other words, the combination "it's" sits very comfortably in the company of:
* That's (is) nice.
* There's (is) a dog in the garden.
* The dog's (has) dug up your bulbs.
The apostrophe here works like an arrow or finger, pointing to the place where letters have been removed. There are really so few complications with this that pupils should quickly understand the principles and get it right every time.
Then there's "its", without the apostrophe. At first sight this is harder to explain because it looks like a good candidate for a possessive apostrophe. After all, it is a possessive: just as the car's owner is the owner of the car, so its owner is it. Pity the poor learner: just when you've struggled hard to get a good firm grip on the possessive apostrophe, you're then told off for using it in a place where it really does seem to be needed.
At second sight, though, things are clearer. Instead of comparing "its" with "the car's", why not compare it with "his"? If you have a male dog, then you're either "his owner" or "its owner". Nobody would dream of writing "hi's", so why put an apostrophe in "its"? Better still, compare it with "her" and "my". Then apostrophes start to look downright silly. These are all possessive determiners, so as a determiner "its" needs no apostrophe. (And incidentally, the same is true of the possessive pronouns such as "hers": "She paid for hers and he paid for his"; here you only need to think of "mine" to see why.) All of this is a sign of the way we sometimes need our pupils to step back and look at underlying patterns and connections between words and word classes. Seeing "its" as belonging to possessive determiners and pronouns - part of the her, my, hers, mine family - will help students to distinguish this use of "its" from the easier "it's". It's another reminder of how easily good grammar teaching can feed clearer thinking and better communication.