Nouns and noun phrases; Source: Wikipedia
The possessive form of an English noun, or more generally a noun phrase, is made by suffixing a morpheme which is represented orthographically as ‘s (the letter s preceded by an apostrophe), and is pronounced in the same way as the regular English plural ending -(e)s: namely as /ɨz/ when following a sibilant sound (/s/, /z/, /ʃ/, /ʒ/, /tʃ/ or /dʒ/), as /s/ when following any other voiceless consonant (/p/, /t/, /k/, /f/ or /θ/), and as /z/ otherwise. For example:
Mitch /mɪtʃ/ has the possessive Mitch’s /ˈmɪtʃɨz/
luck /lʌk/ has the possessive luck’s /lʌks/
man /mæn/ has the possessive man’s /mænz/
Note the distinction from the plural in nouns whose plural is irregular: man’s vs. men, wife’s vs. wives, etc.
In the case of plural nouns ending in -s, and in the case of certain other nouns ending in -s, the possessive is indicated in writing just by adding an apostrophe, and is not indicated in the pronunciation:
the possessive of cats is cats’, both words being pronounced /kæts/
the possessive of Jesus is most commonly Jesus’, both words being pronounced /ˈdʒiːzəs/
Singular nouns ending in -s can also form a possessive regularly by adding -‘s, as in Charles’s /ˈtʃɑː(r)lzɨz/. The Chicago Manual of Style recommends this more modern style, while stating that adding just an apostrophe (e.g. Jesus’) is also correct. The Elements of Style and the Canadian Press Stylebook prefer the form in -s’s with the exception of classical and Biblical proper names (Jesus’ teachings, Augustus’ guards) and common phrases that do not take the extra s (e.g. “for goodness’ sake”). For more on style guidance for this and other issues relating to the construction of possessives in English, see Possessive apostrophe.
More generally, the -‘s morpheme can be attached finally to noun phrases, even if the head noun does not end the phrase. For example, the phrase the king of Spain can form the possessive the king of Spain’s, and the phrase the man we saw yesterday can form the man we saw yesterday’s. This property is taken as evidence that -‘s is a clitic rather than a case ending.
If this makes no sense to you, it’s probably because you understand how to form the possessive of a plural noun and the writer for yahoo.com does not:
The family name’s McCall; the writer probably knew that. What the writer didn’t know was how to form the possessive of McCalls. Here’s a hint: Just add a freakin’ apostrophe!