Witnessed on a train trip to Berlin this last weekend:
Two young men dressed in black hoodies share a two-person seat. They sit with their backs to each other. One is facing the window, while the other hangs his legs over the armrest into the aisle. They talk to each other over the clatter of the train and the blare of the music coming from their mp3 players. The conversation flows though they never once look over their shoulders at the other.