Apostrophes and What They're Not Supposed to Do

I admit to being puzzled by people's confusion about the role of apostrophes. I'll also admit that the apostrophe is quite capable of carrying a large load. Their primary role, however, is indicating possession, either singular or plural.

What amazes me is the tendency of some people, even those who have ambitions to be professional writers, to use an apostrophe incorrectly to indicate plurals. I've seen sentences such as "The boy's and the girl's were studying together." Is this mistake caused by ignorance or carelessness?

I've seen the error often enough to suspect that some people really do confuse plurals and possessives. When they see phrases such as "the boy's backpack," they see nothing wrong with writing "the boy's are back."

I sincerely hope this error is not one that becomes commonplace.