When you're sitting on a train or on a 'plane, or eating on your own in a restaurant, you can't help but look at the people around you. I think humans are innately social beings rather than solitary. You can only glance at them for a second otherwise people start to shift in their seats, because they feel that you are starting to invade their personal space. How often have you imagined what people are like just from that brief observation? If I am bored on a long journey I will often do this. We may use certain clues to help us in this quest, what they are wearing for example, and I have blogged on this before.