It was not a large office, but it was quite spy-proof and quite undetectably so. Spy-beams trained upon it received neither a suspicious silence nor … - Isaac Asimov

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It was not a large office, but it was quite spy-proof and quite undetectably so. Spy-beams trained upon it received neither a suspicious silence nor an even more suspicious static. They received, rather, a conversation constructed at random out of a vast stock of innocuous phrases in various tones and voices.