the mysteries of our streets

Something we take for granted as city kids is that there will always be something new and curious imbedded in our landscape. Sometimes it's temporary, like the giant YOU ARE BEAUTIFUL strapped to a fence off 2nd and Spring Garden Street that slowly lost its letters. 
Some are semi-permanent, like the little pink army men that crawled across the city one summer. They were glued in place, but it appears as though someone pried them loose and took them home. 
Even the more well-affixed symbols have grown legs and walked off- I've heard that if you can peel up one of the pavement robots, known as stikman soon after it's laid down it's a seller's market.
These opposing drives- to create and disseminate street art, and to collect it with photographs or by pocketing it- makes for an ever-changing landscape of urban art.
Have you ever seen these curious plaques in crosswalks all around the city? I thought they were exclusively a Philadelphia phenomenon, but apparently they have popped up in many other cities since 1980 when the first was photographed in Philly. Some may have been placed there by copycats, but most carry the same cryptic message. There's a new documentary called Resurrect Dead: The Mystery of the Toynbee Tiles that explores their history and tries to get to their roots. It looks like quite a nail-biter! Check it out here.