I was amused last night to come up with a sentence where both it's and its were correct. There is an analogue clock in my room which I've been suspecting for a while of going slowly. It was still ticking last night but didn't seemed to have moved at all since the night before (when it seemed to have lost 2 hours in 24). I thought to myself, `I think that the clock's stopped, for all it's ticking' and continued putting books upright on shelves. Then I replayed the sentence and thought, mmmm, that could have been `I think that the clock's stopped for all its ticking'.