The most common apostrophe mistake is confusing its and it’s.
The rule is simple: only use an apostrophe when you are shortening it is.
It’s (it is) a lovely day.
It’s with an apostrophe has nothing to do with possession. Its without an apostrophe is already a possessive noun. Other possessive pronouns are his, hers and ours and nobody uses apostrophes with them.
Remember the cliché:
It’s a wise dog that scratches its own fleas.
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