YSK that “its” is a possessive pronoun while “it’s” is a contraction of “it is”.

Some possessive pronouns do require apostrophes (examples: “one’s”, “somebody’s”), so it can be tricky to remember to leave the apostrophe out of the possessive pronoun “its”. The best memory aid I’ve come up with for this is: “The contraction definitely needs the apostrophe, so the one that doesn’t must be the pronoun.”