Possession isn't the only use for 's; it can also be a contraction for is. For example: it's, how's, he's, she's, that's, etc. In standard English, s' is never a contraction *. The double meaning of 's leads to one of the most common mistakes in written English: confusing its, which is the possessive form of it, and it's, which is a contraction ... More @Wikipedia
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