This one is a request from Sorcha, and I do find I see this quite a lot, so it’s probably time to feature it. It’s one of those ones like their/they’re/there or its/it’s which people seem to know there’s something odd about, but then take a wild stab in the dark and go for the wrong one. It’s all because of the apostrophe, and that feeling that if something belongs to someone, it should have one, isn’t it. Anyway, there is a rule, and I’m going to explain it now. If you know someone who does this one on a regular basis, do point them towards this blog!
Who’s is a contraction for “who is” or “who has”. If you can use “who is/has” in a sentence, you can use “who’s”. “Who’s that girl?”; “Who’s sat on my hat?”; “Who’s going to volunteer to think up some more troublesome pairs?”; “he’s the one who’s always going on about knitting”.
Whose refers to something belonging to, or associated with, a person, or “of whom or which”: “The man whose hat it was became very angry”; “whose girl is that?”, “whose turn is it?” “The dog whose tail was wagging most was awarded the prize”.
So – who is = who’s. Who owns = whose. “Who’s sat on that hat? The man whose hat it is will be really angry!”
You can find more troublesome pairs here and the index to them all so far is here.