BooksForKidsBlog has a lengthy post up about how prolifically people -- even published authors with editors -- are misusing the apostrophe.
So, a short lesson:
The apostrophe only denotes contraction or possession. It can never be used to denote pluralization.
Correct: The Ewoks didn't have enough beer. [contraction]
Correct: John Wesley brought them a six-pack of Killian's Irish Red. [possession]
Incorrect: The Ewok's were pleased. [faulty pluralization]
I remember seeing a preschool advertising itself with an incorrect pluralization in its name. I knew immediately that I would never send any children of mine there during their formative years.
Although I hope that the Klingon-language tapes that we've been playing in utero will give our baby a sound foundation in conceptual grammar. I'm starting her on the Nerf Bat'leth at six months.