" Form the possessive singular of nouns by adding 's. Follow this rule whatever the final consonant. Thus write: Charles's friend, Burns's poems, the witch's malice. ... The pronomial possessives hers, its, theirs, yours, and ours have no apostrophe. Indefinite pronouns, however, use the apostrophe to show possession: one's rights, somebody else's umbrella. A common error is to write it's for its, or vice versa. The first is a contraction, meaning "it is". The second is a possessive. It's a wise dog that scratches its own fleas. "

― William Strunk Jr. , The Elements of Style