The apostrophe has a number of functions. The (‘) shows the possessive case. This is why we write Lord’s (the cricket ground) and not Lords’ or Lords because Lord’s was once the meadow of John Lord. The (‘) also is used to take the place of abbreviations as well as omitted letters.
Some 11+ examinations ask for a written paper where the children may write a story or an account or a description. The children have to learn to proof read their output. Eleven Plus children would be expected to cope with the girl’s dress – as in the dress of the girl. We hope too they will remember that they need an apostrophe if a letter is left out – as in couldn’t. How do we explain won’t? If it is an abbreviation of will not why do we not write willn’t?
As teachers and parents whenever we are short of knowledge or information we always have the invaluable option of saying `because’. Perhaps won’t came about because it rhymes with don’t. Who knows? Who cares, as long as the apostrophe is used in the right place. Just remind your children that if they are writing about a rhinoceros it sounds better to say the horn of a rhinoceros rather than a rhinoceros’s horn. (This information just might come in useful.)