Hey, while I'm thinking of this - help me with 'it's'. I automatically put the apostrophe in every time, and then confuse myself thinking 'Should that be there? No, I should take it out...no I think that's right...'

It's - it is.
It's - as in, belonging, like the dog's collar - it's collar. Right?
Its - as in, humour at its best. Is that right?

Can someone help me out here? Am I even more confused than usual?