On the other hand, we show them only once that the apostrophe is used with an "s" to show possession, and thereafter they include an apostrophe with the "s" at the end of any word, possessive or not. Go figure.

        Evidently, there are many adults who are still experiencing confusion about this simple punctuation mark, as witnessed by the new sign on a Palo Alto schoolyard fence: "DOG OWNER’S - KEEP KIDS CLEAN...." The sign-maker or person who ordered the sign correctly understood that "kids" is plural, not possessive, but missed the mark on "OWNER’S."

        Since parents help their children to write at home, it would be useful for them to have a quick review of the use of the apostrophe:

• It shows ownership (boy’s hat, girl’s book).
   
• If the plural of a noun ends in an "s" or "z" sound, add only the apostrophe (farmers’ plows, cows’ bells).
   
• If there is joint possession, the apostrophe goes with the last name ("Marge and Tom’s children" indicate that the children are both of theirs, as contrasted to "Marge’s and Tom’s children," which means that some belong to one and some to the other.)
   
• It is not used with pronouns its, his, hers, ours, yours, theirs, whose. (It’s means either "it is" or "it has." If you write, "The dog hurt it’s leg," you are saying, "The dog hurt it is leg," or "The dog hurt it has leg," neither of which makes sense.
   
• It is used to indicate missing letters in contractions (don’t, isn’t, wouldn’t).
   
• It forms the plural of letters, numerals, and symbols (the word has six t’s in it).
   
• It is incorrect to use it on a house sign such as "The Smith’s." In this case, you are saying that one person, named "The Smith" owns the house. It is correct to write either The Smiths (meaning "The Smiths live here.:) or The Smiths’ (meaning "This is the home of all The Smiths").