Yeah - Eat THIS grammar fans....
An apostrophe is used to indicate possession.
For most singular nouns, the ending 's is added; e.g., the cat's whiskers.
When the noun is a normal plural with an added s, no extra s is added in the possessive, so pens' lids (where there is more than one pen) is correct rather than pens's lids. If the plural is not one that is formed by adding s, add an s for the possessive, after the apostrophe: children's hats, women's hairdresser, some people's eyes (but compare some peoples' recent emergence into nationhood, where peoples is meant as the plural of the singular people). These principles are universally accepted.
If a singular noun ends with an /s/ or a /z/ sound (spelled with -s, -se, -z, -ce, for example), practice varies as to whether to add 's or the apostrophe alone. (For discussion on this and the following points, see below.) In general, a good practice is to follow whichever spoken form is judged best: the boss's shoes, Mrs Jones' hat (or Mrs Jones's hat, if that spoken form is preferred). In many cases, both spoken and written forms differ between writers.
Compound nouns have their singular possessives formed with an apostrophe and an added s, in accordance with the rules given above: the Attorney-General's husband; the Lord Warden of the Cinque Ports' prerogative; this Minister for Justice's intervention; her father-in-law's new wife. In those examples, the plurals are formed with an s that does not occur at the end: e.g., Attorneys-General. An interesting problem therefore arises with the possessive plurals of these compounds. Sources that rule on the matter appear to favour the following forms, in which there is both an s added to form the plural, and a separate s added for the possessive: the Attorneys-General's husbands; successive Ministers for Justice's interventions; their fathers-in-law's new wives.[2] Because these constructions stretch the resources of punctuation beyond comfort, in practice they are normally reworded: interventions by successive Ministers for Justice.[3]
The apostrophe at its worstAn apostrophe is used in time and money references in constructions such as one hour's respite, two weeks' holiday, a dollar's worth, five pounds' worth. Although it may not be immediately obvious, this is an ordinary possessive use. For example, one hour's respite means a respite of one hour (exactly as the cat's whiskers means the whiskers of the cat).
No apostrophe is used in the following possessive pronouns and adjectives: yours, his, hers, ours, its, theirs, and whose. (Many people wrongly use it's for the possessive of it, but authorities are unanimous that it's can only be a contraction of it is or it has.) All other possessive pronouns ending in s do take an apostrophe: one's; everyone's; somebody's, nobody else's, etc. With plural forms, the apostrophe follows the s, as with nouns: the others' husbands (but compare They all looked at each other's husbands, in which both each and other are singular).
To illustrate that possessive apostrophes matter, and that their usage affects the meaning of written English, consider these four phrases (listed in Steven Pinker's The Language Instinct), each of which has a meaning distinct from the others:
my sister's friend's investments (the investments belonging to a friend of my sister)
my sister's friends' investments (the investments belonging to several friends of my sister)
my sisters' friend's investments (the investments belonging to a friend of several of my sisters)
my sisters' friends' investments (the investments belonging to several friends of several of my sisters)
Kingsley Amis, on being challenged to produce a sentence whose meaning depended on a possessive apostrophe, came up with:
"Those things over there are my husbands." (I'm married to those men over there.)
"Those things over there are my husband's." (Those things over there belong to my husband.)
Have some of that....