Whose

Whose is a possessive determiner and relative pronoun that indicates possession. It is also one of the wh-question words.

Teachers do well to point out the difference between whose and the contraction who's.

Polemical usage
Given that English doesn’t have an equivalent possessive form for which or that, generations of kids at school throughout the 20th century, at least, were hoodwinked into believing that whose had to be substituted by of which when referring to inanimate objects, as in ... a book, the cover of which is green, rather than the more natural, i.e. less-stilted ... a book whose cover is green. This fallacy was based on the so-called "rule" that whose, being the possessive form of who, was only be used when referring to persons.

H W Fowler, in his classic Modern English Usage (1926), and with his characteristic no-nonsense attitude to the use of English stated categorically: “Let us, in the name of common sense, prohibit the prohibition of whose inanimate; good writing is surely difficult enough without the forbidding of things that have historical grammar, and present intelligibility, and obvious convenience, on their side”.

And, as Michael Quinion points out, "authors of impeccable credentials over the past 400 years ... have used whose to refer to inanimate objects, including Shakespeare, Milton, Dryden, Addison, Pope, and Wordsworth".