People take pictures of people. Saying “Cheese” or “Sex” immediately puts an artificial smile on the other person’s face. Taking photos of people from the front is so 19-th Century. You gotta be a professional photographer to make a realistic portrait.
Last summer, while walking to my workplace in Manhattan, I started to pay attention to the backs of people’s moving in the same direction. Most of the backs were boring, but sometimes, I saw a back that made me wonder what’s the face of this person looked like? Typically, the face was less interesting to watch than the back. So I decided taking pictures of selected backs without even bothering looking at their faces.
Check them out slowly. Don’t critique the quality of the photos – I’ve been using iPhone while walking. Just try to imagine how the front-ends of these people look like.
yakovfain.com 
Some folks definitely need to check their backs before walking out of the house…thanks for the reminder. And hopefully you didn’t accidentally run into anyone while holding up your phone and trying to focus 😉
The others ‘ backs are all what we see if we have the habit of following, and the backs are not the most interesting parts to see (though we’re luckier than the gnus which can see even less than the back : they see only the other gnus asses, worse, some in action)