How do I know if it's 'its' or 'it's'?

Okay, this one stumps people because it seems so counter-intuitive. Let's look at a couple of sentences.

The cat licked its tummy.

 It's a black cat.

Many people want to put an apostrophe in that first sentence because, after all, don't we use an apostrophe to show possession? Well, yes, but unfortunately that is where English grammar starts to get ugly because the 'its' in that first sentence is actually a possessive determiner and they don't take apostrophes.

Before you run for the hills, it's okay, I've stopped the heavy grammar talk right there. All you need is one nice easy rule:

If it's short for 'it is' or 'it has' then use an apostrophe. For everything else just leave out the apostrophe.

So let's check the sentences near the start of this post.

'The cat licked it is/it has tummy' is obviously wrong. The sentence must be 'The cat licked its [own] tummy' so 'its' without an apostrophe is the way to go.

 'It is a black cat' makes perfect sense so we can use 'it's' with an apostrophe.

It's that easy!

The Oxford English Dictionary has come a long way.

When I was growing up we had a copy of the Shorter Oxford English Dictionary (two volumes, bound in blue fabric) on our bookshelf. It contained words I had never heard of and would never use. Archaic words and scientific terms were crammed in beside bizarre borrowings from foreign languages. It spelt disaster if referred to for Scrabble.

Forty years have passed and the OED has just released some of the latest words to make it into the dictionary. Is a future generation going to be just as bamboozled by "bants", "hangry" and "brain fart" as I was by "canthus", "dessiatine" and "lacustrian"? At the very least, the addition of new words with the letter "x", "Grexit" and "Brexit", will be welcomed by Scrabble players. Hmm, perhaps best to just stick with The Concise Oxford Dictionary.