Lone Wheel

I was walking down the street the other day and saw this lone wheel sitting at the edge of the pavement, and as I often do I had my camera with me so I snapped a shot. I wonder two things: What was it doing there and why did nobody pick it up. At any rate, I thought it looked interesting enough to share (it's the little things that you see and notice when on foot that make it worthwhile...I think).