It overtook and passed her as she reached the thirty-seventh floor. Moving that way, it no longer reminded her of a backpack, but of the black egg case of an almost-extinct animal called a skate, that she’d seen on a beach in South Carolina, an alien-looking rectangle with a single twisted horn at each corner. Tumbling straight up the building now, in a smooth sequence of sticky-footed somersaults. Caught itself with the two tips of whichever pair of horns, or legs, was leading, flipped over, then propelled itself higher with the pair it had just used to grip the surface.