Way to go! You know not to trust the MS Grammar Check and you know "no" from "know." Now, go forth and spread the good word (or at least, the proper use of apostrophes).
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At least I came out of High School knowing my they're, there and theirs!
Now that I think about it, I probably did pay attention only 69% of the time, what with sports, boys, and teenage drama. I did learn to juggle in PE, though. So at least we know that time wasn't wasted.