who’s / whose

Same trick as its/it’s, and the same 50% failure rate.

Wrong: Who’s turn is it?

Right: Whose turn is it?

The turn belongs to someone. Possessive, so whose.
Wrong: Whose bringing lunch?

Right: Who’s bringing lunch?

“Who is bringing lunch.” Contraction, so who’s.

The test

Say “who is.” If it works, use who’s. If it doesn’t, use whose.

This is structurally identical to its/it’s and your/you’re: apostrophe means a contraction, never possession. Possessive pronouns don’t take apostrophes, and whose is one.

Learn one of these three and you’ve learned all three.

Now go ask whose turn it is.