Where does the apostrophe go in a possession word?

Apostrophes are those little curved marks you see hanging from certain letters. They look harmless enough, so why do even well educated people throw them where they don't belong and leave them out where they're needed? Until apostrophes disappear from English altogether, you can take one step toward apostrophe reform by perfecting the art of showing possession.

my aunt's pen

the lovers' letters

that corner bar's fine wines

All of these phrases include nouns that express ownership. Think of the apostrophe as a little hand, holding on to an s to indicate ownership or possession. In these examples, you notice that the apostrophe is used to show that a singular noun owns something (aunt's, pen; bar's fine wines). You also see a phrase where the apostrophe indicates that plural nouns own something (lovers' letters).

Here's the bottom line: To show possession by one owner, add an apostrophe and the letter s to the owner:

the dragon's burnt tooth (the burnt tooth belongs to the dragon)

Lulu's pierced tooth (the pierced tooth belongs to Lulu)

Another way to think about this rule is to see whether the Word of expresses what you're trying to say. With the of method, you note

the sharp tooth of the crocodile = the crocodile's sharp tooth

the peanut-stained tooth of the elephant = the elephant's peanut-stained tooth

and so on.