[trealira]

To be fair, in most other nouns, adding an apostrophe and an s at the end can both be short for "is" and signify the possessive form (e.g., "Tim's a jerk" means "Tim is a jerk," but "Tim's house" means the house Tim lives in).

By analogy, it makes sense for "it's" to both mean the possessive form and to be short for "it is," but instead we spell the possessive form as "its." Since the difference is only in writing, not pronunciation, it makes sense for a native speaker to forget it.

I see the same mistake with "who's" and "whose." The latter is the possessive form (e.g. "Whose phone is this?") and the former is a contraction of "who is" (e.g., "The only kid who's sitting quietly"). I see people write "who's" instead of "whose," for probably the same reason, since relative pronouns also replace normal nouns. Here's a comparison with the word order changed to make it obvious ("who" replaces "Mom" here):

  This phone is Mom's.
  This phone is who's/whose?

I know the normal word order is "Whose phone is this?" though.

[raverbashing]

> in most other nouns

Yes but it is not a noun

And you don't use it for "his" or "hers" or "theirs"

(So maybe that should be the hint people follow - if you can use "theirs" it's "its", then)