When I moved to US I was at first startled when neighbors and others asked me "How do you do?" while walking past me on the side-walk. Of course it just meant the same as "hello". But I wasn't expecting it. So I started thinking how do I do? And what should I tell them about that?

By the time I had come up with some kind of answer they had already passed me by. I didn't realize I was supposed to answer by asking the same question from them, or some slight variation thereof.