Somebody is knocking on the door outside. The person inside the house says, "Who is it?"

Question

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Why does he say 'Who is it?"
What about "Who are you?" Can't we use "Who are you?" Do you have any difference between the two sentences?

Answer 1

"Who is it?" is just a common expression. If the person inside says "Who are you?" it comes across as a bit insulting. This is one of those idiomatic things that really defies explanation!