Kids who don't listen or badly-taught writers
Frequently suffer from apostriphitis
If you're unsure where 'n apostrophe fits
These are a few of the difficult bits

Simple possessives like "Roger's" will have one
Meaning belonging to something or someone
Plural words (such as "words") as you can see
Do not per se have an apostrophe

"His", "hers" and "theirs" let's see: how THAT lot fit is
"Its" is the same (unless it's short for "it is")
Possessive pronouns are what we call these
And never, oh NEVER have apostrophes

When the pen writes,
When the e mails,
When it don't seem right
Just sing your way through all the difficult bits
And then you'll know what to write

Endings on verbs (such as "goes" or "dismisses")
Of course they don't have them - nor Mr. or Mrs.
"At your wits' end" - means "the end of your wits"
I guess that is one of the difficult bits:

Plural possessives do cause much confusion
Like "children's", i.e. it belongs to the children
If plural words stop with an "s" at the end
Just use an apostrophe - no "s": comprehend?

One last example that often distresses
"Belonging to " words that all finish in "s"s
There ain't no reason why this should cause stress:
You write the apostrophe AFTER the "s"

When the pen writes,
When the e mails,
When it don't seem right
Just sing your way through all the difficult bits
And then you'll know what to write