"Whenever" and "Seen"

[pomtzu]

The one that really bugs me, that I see everywhere these days: people using "it's" when they mean "its."

it's = contraction of "it is"
its = possessive of "it"

I guess I'm always wrong then. I was always taught that an 's after anything, was the possessive of it, as well as being 2 words shortened - where just s is the plural of it. I never knew there was such a word as its. Its would mean that it is plural.