This one is pretty simple once someone points it out to you. Here is the exception to the rule in English about how we make possessives and contractions. We usually make the possessive by adding apostrophe (') s. We make a contraction in the same way. Hence, you would think "it is" and "of it" should both be "it's." But this way confusion lies. So once upon a time someone arbitrarily decided that "it is" has priority and deserves the honor of the apostrophe. "Of it" must therefore be satisfied to live without one. So think of meaning and remember:
it is = it's of it = its
As for "its'," a construction I have indeed seen, try to imagine the sentence that would need a possessive of the plural "its." For the only one I can imagine, the word "its" would have to appear several times on the blackboard in green chalk, and someone would have to be saying something about the many "its" and "the its' color." Other than in that ludicrous stretch, no such word exists.
The Possessive
As long as we are on the topic of apostrophes, let's deal with the possessive. Everyone knows how to make the possessive, but not everyone knows how to distinguish the plural from the singular. It's very easy. If the thing or things being possessed are being possessed by more than one thing, and the corresponding noun has an "s" at the end, the apostrophe goes outside the "s." A student who writes about "my fathers' face" is saying literally that he or she has more than one father and all those fathers share one face. Since most humans can have only one father, it should be "my father's face," or perhaps if the father is the two-faced type, "my father's faces."
Dealing with the plural possessive can get tricky when the singular form of the word ends in "s." If there is only one syllable, simply treat the word as you would any other: "my boss's foot." With two or more syllables, you have problems. And here we reach the horizon of the known world. Beyond this point, truth belongs to the person with the best argument. There is no absolute. For every expert who claims authority for a particular answer, there are two other equally valid authorities with different answers.
