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!