A superstition and a folf belief

Rules Project

I don't understand why  one of my daughter's teachers mentioned to her to use  whose  only to refer to people.    I thought  whose  refers to people as well as animals and things.  So I researched this.   According to     http://www.grammarphobia.com/blog
whose is the possessive of who and which.   Whose can mean of whom or of which.   Also according to the aforementioned website, Fowler's Modern English Usage (rev. 3rd  ed.) by R. W.  Birchfield,   Merriam-Webster's Dictionary of English Usage and the Oxford English Dictionary all concur that limiting the usage of whose to refer to people is "superstition" and a  "folk belief."