How invisible ink works – and other novelty mysteries


It's the stuff of spy thrillers: a James Bond, Mission Impossible gadget you can make in your own kitchen. But how does invisible ink actually work, and how can you make it? Read on to discover its secrets -- and those of a number of other favorite novelties.

Invisible ink

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How old-school is this clever novelty? Very, very old, it turns out. Roman thinker Pliny the Elder is among the first to have written about its use, but it's proved its worth in countless other conflicts over the centuries. Both sides used it during the American Revolutionary War, World War II prisoners used it to send home secret messages, and higher-tech derivatives even found use as recently as the Cold War-gripped 1950s.

So how do you do it? Depending on the security you need, the materials you have, and the amount of time and effort you want to put in, there's a variety of options. Almost any kind of acid will work as invisible ink, as long as it dries clear: gently warm the paper (over a hot light bulb, say) and the heat will oxidize the acid, revealing the hidden message. Lemon juice, vinegar, wine, and milk are all easy-to-source choices.

But that's not going to do a great job of keeping your secrets; applying heat is the first trick an enemy agent will try. More complex inks exploit chemical reactions to do their developing. Write your message in a clear substance, then later coat the paper in a special developer that sparks a chemical reaction, changing the color of the ink and revealing your scrawls. These are harder to improvise at home, though red cabbage water (the liquid left behind after you boil red cabbage) serves as a developer for a number of household chemicals including ammonia and lemon juice, and iodine does the same for starch.

Other invisible inks only show up when they fluoresce under a blacklight. Bodily fluids work here, though in the interests of hygiene we don't advise too much experimentation. Laundry detergents usually contain optical brighteners that will serve the same purpose, though, and commercially available security pens are also worth a go.

X-Ray specs

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You've undoubtedly seen the ads in the back of comic books or magazines promising glasses that shoot X-rays, allowing you to see through other objects...usually accompanied by a lurid picture of a grinning boy gazing at an attractive young woman.

That's a temptation too strong to resist for many young men, who sent off their dollar only to sadly discover that no X-rays are involved in X-Ray specs. Indeed, if they were, you probably wouldn't be able to buy them. You know that heavy lead apron your dental technician makes you wear when you get your molars X-rayed? That's not for decoration -- the cumulative effects of the mild radioactive dose inherent in getting an X-ray are no joke.

[Related: X-Ray Radiation Concern for Kids]

So how do they work? Pull a set apart, and you'll find that the "lenses" contain nothing more high-tech than a feather sandwiched between two sheets of cardboard. As scientists have known since the 17th century, feathers have an unusual optical property: hold one up in front of your eye and look through it, and its closely-spaced fronds will diffract the light, giving rise to a characteristic pattern of light and dark stripes. Put one in front of each eye, and the result is a fuzzy, misaligned sight that's sort-of-but-not-really like an X-ray image.

All is not lost, though. If you really want a machine that can see through people's clothes, put down the novelty ads, pick up the phone, and call the TSA. Perhaps they're hiring.


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Often right next to that X-ray Specs ad is one for Sea-Monkeys. "Enter the wonderful world of Sea-Monkeys!" they blaze. "Own a bowlful of happiness...instant pets!" The classic accompanying picture depicts a simpering family of humanoid creatures relaxing beside what looks to be a beachside castle, no doubt constructed by their mail-order civilization.

Actually, no, as any South Park fan can tell you. Sea-Monkeys -- or, to give them their proper name, brine shrimp -- are a simple crustacean species found in particularly salty water, and grow to about half an inch long at most. They're curious little beasties -- for one thing, they can breathe through their feet -- but their most noteworthy claim to fame is the fact that their eggs can dry out completely and yet still be perfectly healthy. Take a pile of dried eggs, add salt water, and wait: in as little as a week you'll have a colony of mature brine shrimp partying away.

This convenient storage process has made the little critters a favored food source among fish farmers -- so much so that about 2,000 tons of the dried eggs are sold annually. They've even made their way into orbit as part of a science experiment carried on the Discovery space shuttle in 1998.

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