You can call me nitpicky, but I can't stand it when people misuse apostrophes. It is especially annoying when it's done so frequently in the public view, like on signs, flyers and even by teachers occasionally.
Let me offer some basic tips so that you can avoid this very common grammatical error:
The function of an apostrophe is to show possession OR to replace a missing letter in a contraction. For example:
"I am" can be written as I'm
"do not" can be written as don't
the girl's hat - the hat belongs to the girl
teacher's pet dog - the dog belongs to the teacher
Apostrophes are not needed when a word is plural unless it also involves showing possession!
the girl's went outside
the boy's went outside
chair's and table's for rent
These are all incorrect because the word with the apostrophe is just plural, meaning there are more than one girl or boy. These phrases are not trying to show possession so an apostrophe is NOT needed.
If a phrase needs to be both plural and possessive, then the apostrophe goes at the end.
girls' hats - more than 1 girl owning more than 1 hat
knights' armor - more than one knight has armor
If you don't believe me, here is a reference: