The Milky Way To Make Invisible Ink
Put a little milk in a small bowl. Write with the milk on a piece of paper with a Q-tip or a brush. Let your message dry completely. To read the message just heat the paper. Use an iron or 100-watt light bulb or stove element. Don't rest the paper on the bulb. Ask an adult to help in case a fire starts and never use a halogen light.
Why it works: Milk is an organic product which means it comes from a living thing. When it's heated, it burns at a slower rate than the paper. Your invisible message shows up brown.
This works the same way as the Milky Way. Simply dab a Q-tip or brush into a bowl of lemon juice and write away. Just make sure you don't use too much. To see the message, simply heat the paper after it dries. Another way to see the message is put salt on the drying ink. Give it a minute and then wipe the salt off. Use a wax crayon to color over the message.
Why it works: Both lemon juice and milk are mildly acidic and acid weakens paper. The acid remains in the paper after the juice or milk has dried. When the paper is held near heat the acidic parts of the paper burn or turn brown before the rest of the paper does.
These are only a few ways to make invisible ink. Using the same heat method, you can also use white wine, vinegar, apple juice, and even orange juice, to name just a few. Try some other fruit juices to see what works.