Not far from the New York Stock Exchange, a small crowd of tourists stopped to watch as a bomb-sniffing dog checked out a delivery van. The cobblestoned street was blocked by a line of brass cubes with holes that glowed red like the inside of a toaster.

Suddenly an entire section of the street rotated, cobblestones and all. The cubes moved out of the van's way and their holes turned green. The crowd "ahhed" with surprise.