I culled this from a Norwegian poster to a model airplane newsgroup!

It's is not, it isn't ain't, and it's it's, not its, if you mean it
is. If you don't, it's its. Then too, it's hers. It isn't her's. It
isn't our's either. It's ours, and likewise yours and theirs.
-- Oxford University Press, Edpress News

No wonder it's not easy!

Yes. It's true that one needs to keep one's wits about one when discussing whether it's its or it's!

Richard English

Or--'Tis!