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.
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.
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.