Let's say you're a spy, out in the field, spying. You need instructions now and then from headquarters, but you don't want to risk exposure by picking up a phone (tappable) or getting an e-mail (traceable). Face-to-face meetings carry their own risks. What do you do?One solution, dreamed up during the Cold War: Listen on shortwave radio at a predetermined time and frequency for a message that only you can understand. Numbers stations, it turns out, are the one-way chatter of espionage agencies to their spies. This isn't conspiracy theory hokum; it's referenced in a dozen-plus memoirs of assorted ex-spooks and defectors. And though numbers broadcasts might sound low-tech in the age of the BlackBerry, the idea isn't utterly cockamamie."Even if you assume that most of the messages are 'pick up this money' or 'drop off the laundry,' think about what numbers stations represent. The only way a secret like this can be kept is if you live in a society where everybody is obeying and everybody is a little sleepy. But if you're a curious kind of chap you'll wonder, if your government can keep this a secret, what other secrets are they keeping.