I've always loved trying to fake natives of another culture out (I was always good with langauges).

It's the little things -- telling time in Russian, knowing that in German a "pack" of cigarettes is a packet, that one "directs" the pony rather than steers it -- the little colloquialisms that give the person away. I had so much fun with vampires because they would be nearly perfect at mimicking the local color and then all of the sudden something would come at them they didn't expect and they would almost give themselves away!

That's what makes the human children so much fun. :)