Addressing A Stranger
Addressing a stranger with ‘Sir’ is quite common in the US. The only person you address with ‘SIr’ in the UK is your teacher. In the United States, the teacher is the only person you don’t use ‘Sir’ on.
In the United States, your teacher is always Mr./Mrs./Miss followed by their surname. And yet you might find yourself called ‘Sir’ by a random person on the street.