http://garyes.stormloader.com/its.html
>The confusion between it's and its occurs because on virtually every other word 's indicates possession, so English speakers naturally want to use it's to mean "something belonging to it." But it's is only used when it's a contraction of it is or it has.

The ironclad rule - no exceptions - is that if you can replace the word with "it is" or "it has," use it's. Otherwise, it's always its.

Fucking English.