In early elementary school, we were taught that an apostrophe is placed before an 's' to show singular possessive (the cat's meow or the baby's coo).
Why do people -- people under 35 usually -- put an apostrophe before 's' that indicates a plural? (The girl's got two coat's for Christmas or Those kitten's certainly are frisky when they play with ping-pong ball's). That is totally wrong!!