It had to be said.

The apostrophe is almost never, ever, used to make a plural of anything.

There is one, and exactly one, exception to this. The only time an apostrophe ever makes something plural is with individual letters of the alphabet. Not acronyms. Not abbreviations. Not dates. And for Mata Nui's sake, not other nouns.

The plural of "book" is "books." The plural of "MOC" is "MOCs." If you say "book's" or "MOC's," you are describing something belong to one single book or one single MOC.

An apostrophe is always used in the following:

• In a possessive noun. If placed before the s, the noun is singular; if after the s, the noun is plural. For instance, "Brünnhilde's horse" or "the Valkyries' helmets." (The possessive form of a noun ending in s is still still -'s, by the way. Like "Demetrius's pants.") This is different only if a noun is plural without an s at the end, like "the people's opinion."
• In a contraction, whether verbal or nominal. Here the apostrophe denotes a letter that has been dropped, for instance "don't" instead of "do not;" or "she's" instead of "she is;" or to note missing numbers in a date, as in "the '90s;" or in a textual rendition of someone's speech where a letter is dropped, like when Hagrid says "maybe he just liked killin' by then."
• In the plural of a single letter. There are a lot of a's in "a banana," for example.

An apostrophe is never used to:

• make a plural noun.
• make a plural of an acronym or abbreviation, like MOCs or ACTs.
• make a plural of a date, like saying "the 1990s." There is no apostrophe there.
• (yes, I actually have seen people do this) make the third person singular of a verb. It is "he runs," not "he run's."

I should differentiate here between "its" and "it's." "Its," with no apostrophe, is the possessive adjective form of "it." (Much like "his" or "her.") "It's," with an apostrophe, is exclusively a contraction of "it is" or "it has" or "it was." And for the record, it is spelled "hers," not "her's."

If I catch you doing any of that second list, or some other abhorrent travesty of punctuation, I will set the militant wing of the Apostrophe Protection Society on you. And Mata Nui only knows what would happen then.

~ ToM