I was working at a music store before that also offers music lessons, and most of our customers are half-chinese. I'm not a Chinese citizen but for some reasons, I look like one (maybe because of my monolid eyes). Anyway, it happened once that a parent came up to me and talked to me in straight chinese, and then she smiled. Of course, I didn't know what to say! I was quiet for a while and then I politely said, "I'm sorry?" She did not respond. Later on, I learned that she can't understand English. That was humiliating.