A note on apostrophes in the possessive

Everyone knows how to create the possessive using the apostrophe, right?

If the apple belongs to John, it's John's apple. If the apples belong to the teachers, they are the teachers' apples. When the possessor is singular, the possessive is created by adding an apostrophe then an S, but if it is plural, the apostrophe is added after the pluralizing S. Simple, easy, everyone knows that. "What do you think we are, Sugar Kat?" I can hear you saying to your monitor. "Retarded? 'Cause we're not."

I know you're not "retarded," Dear Reader. However, I have a question for you. How does one create the possessive using an apostrophe when the possessor is singular but ends in an S? How does one write the apple of Mr. Thompkins using the possessive apostrophe?

National magazines and highly-respected newspapers are getting the answer to that question wrong, which is why I feel compelled to write this.

A lot of people for whom English is their primary language would claim that one creates the possessive form with a singular, S-ending possessor the same way one would with a plural, by simply putting the apostrophe after the S. When told the apple belongs to Mr. Thompkins, they write "Mr. Thompkins' apple." They are wrong.

I know this because my mother's maiden name ends in an S and, in her day, kids were actually taught grammar in grammar school. (How I wish I was taught the elements in elementary school, but no, I had to wait until high school. Man, this country's backwards when it comes to when kids should be taught what.) She passed her knowledge of the use of the possessive apostrophe on to me. Now I pass it on the the English-speaking world at large, which has been pissing me off lately with its ignorance of the English language. (Grr!)

If you have a possessor which is singular yet ends in an S, you treat it as you would any other singular possessor, by adding the apostrophe and then the S. Therefore, if the apple belongs to Mr. Thompkins, it is Mr. Thompkins's apple.

So, Entertainment Weekly, if the mirror belonged to Sirius, it would not have been Sirius' mirror, unless you were somehow actually writing about a multiple of whatever the hell Siriu might be which happened to own the same mirror. No, what you should have written, or what your apparently very lax editor should have corrected it to when you made your error, you dim-witted hack, was Sirius's mirror. Sirius is not a multiple, but one character. And your entire magazine is staffed by English-language 'tards.

Proof
As for proof of this, please take out your copy of Strunk & White, The Elements of Style. In the fourth edition, the rules on possessive singular of nouns is on page 1. If you don't have a copy at hand, go and buy one. It really should be required reading for anyone who wishes to write in English. Also, it's inexpensive, so it's not like buying some other grammar and style books, which can put you back $20 to $40. However, if you're just looking for the rule, Bartleby to the rescue: They have posted William Strunk, Jr.'s original book upon which White expounded and expanded. The page you can find the illustration of the proper use of possessive apotrophes is here. Please note the first two samples end in S (Charles, Burns) and still have the apostrophe S to form the possessive.