Animals have some kind of instinct to warn them of danger in the air. People like Rachel and I had it too. It's a sort of familial code, a dog-whistle signal so obscurely pitched, so far off any conventional tonal scale, that even those NASA computers monitoring sounds in space couldn't pick it up. What Rachel had said had left a residue of something chemical in the air, a kind of static that set off a chain reaction, which forced the ions in the atmosphere to flow back on themselves, like a hand brushing suede against the grain.