I was with a colleague in a hotel in Zürich, waiting in line for the breakfast büffet, when two smartly-dressed German businessmen gave us a dirty look and commented that the hotel let in far too many American tourists. Since we were also there on business, and not for tourism, I spoke up and remarked in perfect German, "yeah, some of whom happen to speak fluent German." Totally embarrassed, the one who made the remark, came back with a lame, "well, I didn't really mean it like that."
One time, my younger daughter was at an after-school day care center at her elementary school here in Germany. My wife was busy with something and I was home that day, so she asked me to go pick our daughter up. I said sure, and went to get her. When I got there, I told her to pack up her things and get her coat. I always spoke to our girls in English in an effort--successful, by the way--to raise them bilingually. So, when I went to get her, I spoke in English, as we always did. A Turkish kid heard this, and wondered what was going on.
"What was that?" he asked in German.
"English," I answered back in German.
"She knows English?" he asked in awe.
"Of course she does," I answered.
Puffing up his chest, he said, "well, I know something you don't know."
"Could be," I answered. "What would that be?"
"Turkish!" he said, proudly.
I answered, "Sen ne söyledin? Ben çok iyi Türkçe konuşyorum!" (What did you say? I speak Turkish just fine!)
The kid was so shocked, that he answered (luckily in German--my Turkish in reality is minimal), "how come you know that?"
I responded, also in German, "how come you automatically assume I DON'T know that?"
The teacher on duty, never having seen this kid at a loss for words, ever, admonished him, "you see, Zarkan? You should never assume things about people you don't know."
I doubt I caused some long-lasting fundamental behavioral change in the kid, but he DID stare at me open-mouthed as my daughter and I left for home. The teacher told me later on that it was the only time she had seen this kid at a loss for words.