Mistakes with these two words are very common, but there is a fool-proof way of checking to see if you’ve got the right one:

It’s is short for it is or it has and nothing else. Therefore if you can’t substitute it is or it has in your sentence, you’ve no doubt got it wrong, and you need its without an apostrophe which is a possessive adjective (like his and her).

Look at the following examples – in all of them, you can substitute it is or it has for it’s:

• It’s been raining for a week, but tomorrow it’s likely to be sunny.
• It’s a well-known fact that we get wiser as we get older.
• It’s been so good to meet you; I hope to see you again soon.

And compare them to the following where you cannot substitute it is:

• I have to mend my bike because its back wheel has a puncture.
• The bird displayed its bright feathers in an effort to attract the female.
• I want to write to the bank; do you know where its head office is?

Because it’s so easy to make a mistake, it’s really important to read over what you’ve written. The spellcheck helps sometimes which is good, but if you remember the simple rule above, you’ll never go wrong.