Invisible ink facts for kids

Invisible ink is a special kind of writing that you can't see right away. It looks like nothing is there when you write it. But later, you can make it appear using different methods. Invisible ink is a cool way to hide messages, like a secret code. This is called steganography. It has been used for a long time, especially in espionage, which means spying! People also use it for things like stopping fake money, marking their stuff, or even in fun games for kids.

How to Use Invisible Ink

Invisible ink can be applied to paper using different tools. You might use a special stylus, a fountain pen, a toothpick, or even your finger. After the ink dries, the paper should look completely blank. It should feel just like the rest of the paper.

When you send a secret message, it's a good idea to write a normal message over the invisible one. If you send a blank piece of paper, people might get suspicious! It's best to use a ballpoint pen for the normal message. Fountain pen ink might smudge if it crosses the invisible ink, which could give away your secret. Also, try not to use invisible ink on lined paper. It might change the color of the lines.

Fun with Invisible Ink Pens

You can buy toy invisible ink pens that have two ends. One end writes with the invisible ink. The other end helps you develop, or show, the secret message.

Invisible ink is also used in some children's books. These books often have parts of pictures or text printed with invisible ink. A "decoder pen" comes with the book. Kids can rub this pen over the invisible parts. This reveals answers to questions or completes missing parts of pictures. It's a fun way to play and learn, especially when traveling!

Invisible Ink and UV Light

Some special pens use ink that glows under ultraviolet light, also known as UV light or a black light. These inks are totally invisible in normal light. You can only see them when you shine a UV light on them.

These UV-visible inks are often used to mark valuable items. This helps police identify stolen property. For example, the United States Postal Service uses UV-visible ink. They print special codes on envelopes. These codes help machines sort mail faster without cluttering up the address you can see.

Sometimes, artists use invisible ink in their artwork. They might use it with other special paints that react to UV lights. This creates cool effects that change when different lights are shined on them.

Types of Invisible Ink

Many simple household liquids can be used as invisible ink. These are often organic substances. This means they come from living things. When you heat them, they change color, usually turning brown. Any acidic liquid can work for this type of "heat fixed" ink. To make your secret message even harder to find, you can dilute the ink with water. Just add enough water until it's almost too hard to develop.

Here are some common liquids you can use:

• Milk
• Lemon, apple, or orange juice
• Onion juice
• Sugar solution
• Diluted honey
• Diluted cola drink
• Vinegar or wine
• Soap water

How to Reveal Heat-Fixed Ink

To make the writing visible, you need to heat the paper. You can do this in a few ways:

• Place the paper on a warm radiator.
• Carefully iron the paper.
• Put it in an oven on a low heat.
• Hold it near a 100-watt light bulb. This is a safer way to heat the paper without damaging it.

Images for kids

• 

  A coded letter from Benedict Arnold, originally written in invisible ink. Handwriting by Peggy Shippen Arnold is mixed with Arnold's secret message.

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  A US$20 bill showing a visible stripe under an ultraviolet light.

See also

In Spanish: Tinta invisible para niños