Among other usage errors, your spell-check won't distinguish between the two versions of "it" with an "s" attached.
"It is" is contracted to "it's". Pronouns get special treatment for possessive, so that form for "it" is "its". The rule is to remember that we don't spell the possessives of any pronoun with an apostrophe---his, hers, ours, theirs, whose, yours, (and mine, for an outlier).
An explanation for why pronouns are treated differently could be that for regular nouns the plural is just adding an "s", but for pronouns you get a different word altogether (him vs them, me vs us). So we can have a different form for the possessive without confusing it with plural. Since pronouns are the most common nouns, it makes sense to have special rules for them.