I started to talk (in English) and introduced myself to one of the new PhD Students who is French, and after few sentences (and probably because my English is tainted with that cute French accent which never leaves me) he asked me whether I am French. I replied "Yes, you are right I am French", he looked sooooo happy and then started to speak to me in French, but then I cut him and said "... but I am never going to speak French to you".
He was very puzzled. I don't think he knew that there could be one French person in the world who would willingly decide to speak English when he can speak French instead. If I wasn't so nice, he could think that I was trying to be rude to him :-)