This is a photo taken in one of the highest crime neighborhoods in our city. The residents are attempting to take the neighborhood back and this is one of the signs posted in the "hood". This has to do with a drug called crack cocaine. The dealers package the crack cocaine  in what is called "rocks". The "rocks" are actually not much larger than a large pebble.  When the Police pull up, the dealers drop their drugs ("rocks of crack cocaine") , hence the saying, "Watch out for Falling Rock"

This is a close up of the sign.