Punctuation Pet Peeve

People, I am no expert on grammar, but please double check your definitions of "It's" and "Its."
Here are some examples of correct usage:

It's the end of the world as we know it. (It's = It is)

Every rose has its thorn. (Its = Possessive form of it)

Thank you.

UGA is selling a t-shirt with this grammar error, which is very funny but doesn't surprise me:

Picture of "Every dawg has it's day" t-shirt

Also visit this web site dedicated to "preserving the correct use of this currently much abused punctuation mark."

Apostrophe Protection Society