'Resurrect Dead' unlocks a graffiti mystery

One interesting thing about living in a city is noticing all the street art. True, much of it just litters walls and sidewalks with nonsense -- but some of the images tell a story.

That's definitely the case with something called the "Toynbee tiles," a series of mysterious plaques that someone has left in cities all over the country, including Philadelphia, New York, Cleveland and Boston.

I first...