I know, I know, I should resist saying this. But every time I see it, I wonder why it happens, and I should just get this off my chest.

The difference between "its" and "it's" is something that many people, even native english speakers, seem to miss. Yet, it's so extremely simple that I, a non-native english speaker, have been baffled about that common mistake for as long as I can remember.

The apostrophe (') in any sentence usually means that something at the location of that apostrophe is gone out to lunch. In this particular case, it means that the " " and the "i" in the phrase "it is" were hungry. So rather than "it is", we contract that to "it's", and allow the space and the i to enjoy their meal while the apostrophe keeps their seats warm.

Practically, what that means is that every time you want to write "it's", you should consider whether you can replace it with "it is" without making the sentence sound like junk. If you can't, you probably meant to write "its" rather than "it's".

For instance, consider the following sentence:

"It's not possible to repair this car within the budget that its owner wants to pay"

It's perfectly possible to say "it is not possible" here, so we need to have the apostrophe keep a seat warm for the space and the i.

It makes no sense to say "it is owner", unless you're trying to speak a much deformed form of english, so that makes it a possessive pronoun (similar to "hers", "his", "theirs", etc) and you shouldn't use an apostrophe.

Speaking of food, it's time for lunch now.