The apostrophe causes problems with these two homophones.
Whose is a possessive pronoun.
Incorrect: Who’s car is this?
Correct: Whose car is this?
Who’s is the contraction of who and is. Again, the apostrophe (like with it’s) indicates the omission of a letter.
Incorrect: Whose on first using who’s glove?
Correct: Who’s on first using whose glove?
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