Secrets the Size of a Sand Grain

Most secret codes work by scrambling a message so the enemy can't read it. But what if you hide the fact that there is a message at all? This trick is called steganography. If an enemy spy doesn't know a secret exists, they won't even try to crack it.

Movies make invisible ink look easy—just write in lemon juice and hold it over a candle! But during World War II, military censors easily busted lemon-juice spies by swabbing mail with chemical detectors. Real secret agents had to wear surgical gloves and use complex formulas that dried perfectly clear.

The ultimate hiding spot was the microdot. Perfected by German spies, an agent would type a full page of intelligence, photograph it, and use special lenses to shrink the image down to one millimeter—the size of a single grain of sand.

They printed this tiny photo on clear film, punched it out, and glued it perfectly over the period at the end of a sentence in a completely normal letter. Censors scanning the mail saw nothing but friendly chatter, missing the massive secret sitting in plain sight.