The apostrophe is used to show ownership. Most of the time, it presents no confusion:
Bob's bassoon
The woman's finger
My son's toys
The tricky part is using an apostrophe when the owner is plural.
RULES FOR APOSTROPHES
1. If the plural noun doesn't end in -s, add an apostrophe and -s, like above. (This is the easy part.)
the car's axels
the bacteria's growth
the mice's hairballs
2. If the plural noun ends in -s, just add an apostrophe.
the babies' bottoms
the horses' hooves
the politicians' promises
3. If the word is a proper noun that ends in -s, add an apostrophe and an -s. (This is the part people get wrong). Use ONLY with proper nouns. All other plurals should follow the rule above.
Yeats's poem
Ross's riddle
Chris's crisis
