The rule that breaks the pattern
"It's" with an apostrophe always means "it is" or "it has." "Its" without an apostrophe is the possessive form, meaning "belonging to it." This confuses people because we normally use apostrophes to show possession ("the dog's bone"), but possessive pronouns never take apostrophes.
Consider the pattern: his, hers, ours, yours, theirs, its. None of these possessive pronouns use an apostrophe. "Its" follows the same rule, it just happens to look like it should have one.
The expansion test
Just as with your/you're, try expanding to "it is" or "it has." If the sentence works, use the apostrophe. If it does not, leave it out.
- "It's raining" expands to "It is raining." Correct with apostrophe.
- "It's been a long day" expands to "It has been a long day." Correct with apostrophe.
- "The cat licked its paw" does not expand to "The cat licked it is paw." Correct without apostrophe.
- "The company raised its prices" does not expand to "The company raised it is prices." Correct without apostrophe.
Why even good writers get this wrong
English uses apostrophes for both contractions ("can't," "it's") and possessives ("Sarah's report"). These two rules collide at "its," creating a genuine conflict in the pattern. Your brain wants to add an apostrophe for possession because that is what you do for nouns. But pronouns play by different rules.
If you find yourself hesitating, that hesitation is normal. Just run the expansion test and move on. Over time, the correct form will become automatic.