Invisible Ink the Baking Soda Way

Mix about 2 TB spoons of baking soda 2 TB spoons of water. Next, write using a Q-tip, toothpick or brush on a piece of paper. Let it dry completely. To read the secret message, paint grape juice across the paper with a paint brush or a sponge. Don’t forget – grape juice stains.
Why it works: Grape juice has an acid that reacts with the baking soda. A different color appears wherever the secret message is written.

Invisible Ink the Milky Way

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.

Invisible Ink the Lemon Way

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.

Tips: 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.