That little apostrophe causes so much trouble for so many people. But surely it’s not that hard to distinguish the contraction it’s (one word formed from two, it is) from its counterpart, the possessive its.
Perhaps the confusion arises because possession is sometimes indicated by ‘s. What belongs to me is writerguy‘s, for instance, but how many apostrophes do you see in my, mine, your, yours, his, her, hers, our, ours, their, theirs? If you look at it this way, its is not an exception to the rule after all.