Popular linguistics and language blog Language Log latched on to a post on a “grammar fail” blog that featured a screenshot of one of my grammatical pet peeves: a confusion of its and it’s.
As the more grammatically aware will know, the words get confused often due to the fact that apostrophes usually imply possession (e.g. “This is Fred’s shoe”) or missing letters (e.g. “I didn’t do it”). In the case of its and it’s, one implies possession and the other is short for “it is”, so it’s not surprising that some people forget the rule of which one to use in a given situation. The way I’ve always remembered it is that his and hers – also third person possessive pronouns – do not have apostrophes, so neither should its. It’s is always short for “it is”.
Anyhow, the “grammar fail” quote in question is “milk it for all it’s worth”. While its is usually used in this context, this Language Log post nicely sums up why “it’s” can also be perceived as the correct form to use, whether the author intended it to be or not:
[...] Fiona Hanington pointed out to Language Log that it’s not necessarily a fail. It’s the wrong spelling if worth is the noun meaning “value”, so the intended meaning was “Milk it for all the worth (= value) that it has.” The genitive pronoun its is not spelled with an apostrophe; the right spelling would be Milk it for all its worth. However, there’s another meaning, where worth is an adjective: it could be intended to mean “Milk it for all that it is worth.” And there the apostrophe would be correct (indeed, required): Milk it for all it’s worth. (English is loaded with little gotcha things of this sort, isn’t it?) Since both mean roughly the same thing (they put it in different ways, but it’s hard to imagine one of the meanings making a true claim where the other didn’t), Fiona is right to note that this is one of the very rare cases where it’s and its are both correct in the same context with the same meaning. You won’t find many of those.
Such instances of both forms being correct are rare, but as this goes to show, they do still happen.