The word “its” is a difficult and weird word since it’s the possessive case and possessive noun clauses usually do end with ‘s, like “Mary’s chain”. We don’t have anything similar in Swedish, either (we would write “Marias kätting”), so I have no intuition for this and it’s one of my most common English mistakes. I try to remember it by thinking that the other possessive pronouns don’t have apostrophes either, it’s “his”, not “he’s”.
I still make these mistakes and have been thinking of an Emacs mode to just highlight all “it’s” no matter what. (The other way around, highlighting “its”, is way less necessary, that error is not as common.)