It’s supposed to be simple. The word “its”—the possessive form of it—is spelled without the apostrophe because it’s a pronoun, even though it is a regular possessive. That’s the rule: nouns spell their possessives with an apostrophe, pronouns don’t. Simple and sensical, isn’t it? Do you agree with this rule? I bet you do, because every single person I’ve had this discussion with has thrown this argument at me. “‘Its’ is a possessive pronoun,” they say, “so it should be spelled without an apostrophe like all other possessive pronouns (my, your, his, her, our, and their).”
Problem is, the rule’s wrong. Most pronouns do spell their possessives with an apostrophe. To wit:
Someone's hat is on the chair. One's mind must always be focuses. Whoever's coat is in the hall better pick it up.
By the rule above, these possessive pronouns should be spelled without apostrophes.
“Oh”, but you say, “it’s not any pronouns; it’s just personal pronouns you don’t use the apostrophe with. All other pronouns you use the apostrophe. It’s still a simple rule.”
And it’s still wrong. It doesn’t account for the word “who”, which is not a personal pronoun yet spells its possessive (“whose”) without an apostrophe. The only way to account for both “it” and “who” is to make a complex rule, one with at least two conditions.
“Ok,” you argue, “but it’s still a pretty simple rule: don’t use an apostrophe for personal pronouns or interrogative pronouns.”
Don’t forget that “who” can also be a relative pronoun.
“…or relative pronouns.”
Now it has three conditions, and it’s still wrong. To wit:
The computer that's turned on is wasting power.
True, it’s probably not the best style, but it’s perfectly intelligible and would not ever be spelled without the apostrophe. (And don’t give me any Chomsky bull about “that” being a relativizer here and not a relative pronoun; if it were a relativizer this sentence wouldn’t be intelligible.)
“Ok, fine”, you say, “everything except ‘it’ and ‘who’ uses an apostrophe.”
Now you’ve forgotten about the other personal pronouns.”
“Everything except personal pronouns and ‘who’ uses an apostrophe.”
Ah, finally we have a rule that works. It’s a little ugly because one condition is a class of words and the other is a singular exception, but it’s relatively simple and no actual reader or writer would have difficulty applying it. It’s not the end of the universe. But the thing is, it’s needlessly complex. If we could spell “its” and “whose” with apostrophes, then we would only need a simple rule with one condition: a word who’s possessive is regular spells it’s possessive with an apostrophe.
But, English orthography, being what is is, has to make things complex for us, even simple things like spelling possessive pronouns.