I've been taking pictures of the backs of people's heads on my daily commute between the suburbs and the city (to and back!) for a little over a month. I run the pics through a simple filter and post the resulting images to Twitter.
Why do I do this? Well, one day I happened to notice the crazy, heaped-up hair of the woman sitting in front of me, took a picture of it, then just kinda fell into taking these kinds of pictures on a regular basis.
I suppose you could argue that taking these pictures could be considered an invasion of privacy, but I feel like the anonymity of the person counteracts the fact that I am taking their picture. Well, I hope it does, at least. I haven't known any of the people in these pictures, and I hope no one takes offense.
I have found that I like picking out a particular characteristic in the world around me, then focus on the characteristic and highlight it (e.g. negative space signage, sidewalk liquor). I don't necessarily doggedly pursue these things that I notice, but I do focus some attention on them.
So, for now, backs of heads it is, then.
Click for bigger.