How to Make Your Own Invisible Ink
Making invisible ink to write and reveal secret message is a great science project to try if you think you don't have any chemicals. Why? Because just about any chemical can be used as invisible ink if you know how to use it!
WHAT IS INVISIBLE INK?
Invisible ink is any substance that you can use to write a message that is invisible until the ink is revealed. You use the ink by writing your message with it using a cotton swab, dampened finger, fountain pen, or toothpick.
Let the message dry. You may want to write a normal message on the paper so that it doesn't appear to be blank and meaningless. If you write a cover message, use a ballpoint pen, pencil, or crayon, since fountain pen ink could run into your invisible ink. Avoid using lined paper to write your invisible message, for the same reason.
How you reveal the message depends on the ink you used. Most invisible inks are made visible by heating the paper. Ironing the paper or holding it over a 100-watt bulb are easy ways to reveal these types of messages. Some messages are developed by spraying or wiping the paper with a second chemical. Other messages are revealed by shining an ultraviolet light on the paper.
WAYS TO MAKE INVISIBLE INK
Anyone can write an invisible message, assuming you have paper, because body fluids can be used as invisible ink. If you don't feel like collecting urine, here are some alternatives:
Heat-Activated Invisible Inks
Iron the paper, set it on a radiator, place it in an oven (set lower than 450° F), hold it up to a hot light bulb.
Inks Developed by Chemical Reactions
These inks are sneakier because you have to know how to reveal them. Most of them work using pH indicators, so when it doubt, paint or spray a suspected message with a base (like sodium carbonate solution) or an acid (like lemon juice). Some of these inks will reveal their message when heated (e.g., vinegar).
Inks Developed by Ultraviolet Light (Black Light)
Most of the inks that become visible when you shine a black light on them also would become visible if you heated the paper.