Oy

I'm 'oying' myself for this post but I feel it's a service so I will sacrifice myself:

'...if you can replace the word with "it is" or "it has", then the word is it's:

It's a long way to Tipperary.

If you can replace the word with "who is" or "who has", then the word is who's:

Who's that knocking at my door?

If you can replace the word with "they are", then the word is they're:

They're not going to get away with this.

And if you can replace the word with "there is", the word is there's:

There's a surprising amount about the apostrophe in this book.

If you can replace the word with "you are", then the word is you're:

You're never going to forget the difference between "its" and "it's".'

- From Eats, Shoots & Leaves by Lynne Truss