My fly-by-night rule for "it's" verses "its":

If you want to make the contraction for the tow words "it is", then "it's" is correct. The apostrophe replaces the missing "i" in it is, contracting the word to "it's" ((it (i)s)). An example would be: "It's about to fall apart". (It is about to fall apart).

If you want to imply ownership, "its" is correct. An example would be: "See the big elephant next to the baby elephant? I think the big one is its mother"

And if we're splitting hairs, an inanimate object cannot "own" anything. So--supposedly--you shouldn't write: "The iron plate is held on by its nuts and bolts". It would be better to write: "The iron plate is held on with nuts and bolts".

Hope this helps.

--P.G.