Jason Georges/KPCC

First in line at the Glendale AT&T store, iPhone 5 owner Mark shows off his shiny new gadget.

More than five million iPhones have been sold since the new model hit the stores last Friday. To put it in perspective, that comes out to one phone for every man, woman, and child in the state of Colorado.

But what happens to the old iPhones when people upgrade? That's the question CNET senior writer Jay Greene set out to answer in his three-part series on the lifespan of the iPhone.