Who’s eating whose pizza?
Someone’s eating someone’s pizza. (or her/his/their)
My cousin’s eating my cousin’s pizza. (or her/his)
Alex’s eating Alex’s pizza. (or her/his)
I’m eating my pizza.
You’re eating your pizza.
She’s eating her pizza.
He’s eating his pizza.
It’s eating its pizza.
We’re eating our pizza.
They’re eating their pizza.
Who’s and whose, you’re and your, it’s and its and they’re and their (and there) are some of the most easily confused pairs (and trio) of words in English, and dominate examples and discussions of ‘grammar fail’ on the internet. It is easy, tempting and, for some people, irresistible to say “Gotcha” and cast aspersions on the writer’s intelligence and/or education, but language is never pure and rarely simple, so it is worth taking the time to consider all the issues.
The first element in each sentence is a contraction of a subject noun or pronoun and a present simple form of the verb be. It is always possible to spell and speak these in full (I have written it is several times already – did you notice? I fact, I have not contracted every possible contraction throughout), but it is usual/natural to contract them, especially in speech. The third element is the possessive form of the same noun or pronoun.
In speech and writing, someone’s, my cousin’s and Alex’s are the same because indefinite pronouns and noun phrases (including names) do not have a separate possessive form. The only possible problem is the omission of one or both apostrophes. In speech, who’s and whose and it’s and its sound the same, and while you’re can be pronounced more like you-’re and they’re can be pronounced more like they-’re, they are usually pronounced the same as your and their (and there). He’s can be /hi:z/ in careful speech, but is usually /hɪz/ in casual speech, the same as his. I’m and my, she’s and her and we’re and our are completely different.
Yet in speech, there is never (or hardly ever) any doubt as to what anyone means. /hu:z i:tɪŋ/ is almost always Who’s eating and /hu:z pizza/ is almost always whose pizza. I can create sentences with the opposite grammar, but they would be very rare in real life.
In writing, also, there is never (or hardly ever) any doubt as to what anyone means. People manage to communicate despite mixing who’s and whose etc.
So which way round do these mistakes happen? Do people more often write who’s when mean whose, or do they write whose when they mean who’s? Note that it’s and its differ only by the apostrophe (and it is never its’, as I have seen on one public plaque), while who’s and whose, you’re and your and they’re and their (and there) have other spelling differences.
There are two opposite factors. The first is that, in general, people prefer the version without the apostrophe (whose, your, its and their), especially on smartphones, where the apostrophe is tucked away on the second level of the keyscreen. The second is that, in general, people prefer the version with the apostrophe, because of the required use of an apostrophe in possessives of indefinite pronouns and noun phrases (including names).
Native speakers over the age of three would never say or write the equivalent of She’s eating she’s pizza, but in my experience many second language learners from China do. Maybe they also say the equivalent of He’s eating he’s pizza, but the result is indistinguishable from He’s eating his pizza. But they never say the equivalent of I’m eating I’s pizza, We’re eating we’s pizza or They’re eating they’s pizza.
My advice: if in doubt, write it out. It is always acceptable to say or write Who is eating whose pizza?
The main reason I try not to say “Gotcha” and cast aspersions on anyone because they have mistaken it’s and its is that I have mistaken it’s and its. At least once. Hopefully only once. In the diary I kept and mostly posted online on a travel blog during my first stay in South Korea (2006-2009) I wrote “[A student] said ‘Pluto was removed from the solar system last year’. I had visions of a giant hand plucking Pluto out of it’s orbit.” None of my readers wrote “Gotcha”. In class I will tell you that it is wrong and explain why, and when subediting a magazine, I will change it, but in real life, I will keep my mouth shut and my fingers not typing.