When to use Its or It's

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Because I'm away from all things computer, I'm keeping the blog going with scheduled posts about the language we use as writers. I hope you'll forgive for failing to reply to comments for the time being, but I'm having a well-earned break from work.
When to use Its or It's 
its: the possessive form of it – this possessive doesn't use the apostrophe, as you would for a named person, but follows the convention for other pronouns, e.g. your - yours, their – theirs.  'The dog wandered into the kitchen, wondering where its dinner might be.'
it's: the contraction for it is or it has.  'I'd say it's an ill wind that..., but it's been said before.'