Today’s Wonder of the Day was inspired by Julia. Julia Wonders, “How do invisible ink pens work?” Thanks for WONDERing with us, Julia!

ATTENTION, SECRET WONDER AGENTS: Today's Wonder, should you choose to explore it, is one of great mystery…and science. Your challenge is to leave a written message on a piece of paper in a way that people won't see the message, even if they stare right at it.

What do you think? How would you answer the challenge? It can be fun to think like a spy and figure out ways to send secret messages. One way to do this (and answer the challenge) is to use invisible ink.

One very simple form of invisible ink is heat activated. These inks can be made from many types of organic liquid. Once the message dries on the paper, it is invisible.

When the ink is exposed to heat, such as under an iron or over a candle flame or 100-watt light bulb, the acids in the liquid turn a different color and the message becomes visible. You can find many heat-activated invisible inks right inside your kitchen, including clear soda, lemon juice, apple juice, onion juice, milk, soapy water, sugar water, and vinegar.

Another type of invisible ink is chemically activated. Chemically-activated inks can be made from any two chemical substances that are colorless until mixed with one another. When the message receiver adds the second chemical developer, the chemical reaction between both substances causes the color to change and the message to appear. Vinegar alone can be heat-activated, but if you add red cabbage water instead of heat, it becomes a chemically-activated invisible ink.

A third kind of invisible ink is a favorite of amusement parks. Long ago, if you needed to leave an amusement park to go to your car, they would simply stamp your hand. When you returned to the park gate, as long as you had the stamp, you didn't have to pay to come back inside. These ink stamps were easy to copy and allowed people to sneak in without ever paying for a ticket.

Today, many amusement parks use light-activated invisible inks. If you need to leave the park, they will stamp your hand, but you may be surprised when you look down to find nothing is there. When you return to the park, they will ask you to put your hand under a special light, and just like magic, a glowing stamp imprint appears.

Light-activated inks work because they contain substances that glow under different types of light, but remain invisible to the naked eye. This means no big, ugly stamp marks on your hand and no more free admission for anyone trying to sneak in. Examples of light-activated invisible inks include laundry detergents containing brighteners, soaps, and human saliva.

Invisible ink may not be obvious to the naked eye, but pen scratches and abnormalities on the paper can be telltale signs of its presence. Some inks are easier to develop than others, which means anyone with knowledge of invisible inks may be able to decode your message.

For example, let's imagine you use soapy water to write your message. If someone found your paper, they might use heat, chemicals and ultraviolet light. Since soapy water is reactive to each of these, it wouldn't take them long to reveal your top-secret message.

Wonder What's Next?

Tomorrow’s Wonder of the Day is full of hot air. Hope you’re not afraid of heights!