Historically, spies have been known to secretly send messages using a wide variety of methods. Some of these messages are encrypted, readable only to those able to decipher them. Other messages are invisible to the naked eye. One such method is the ability to write messages inside of an egg. The egg seems normal on the outside, but will reveal a message when its shell is broken.

Place the egg in small saucepan.

Fill the saucepan with water and cover it with a lid. There should be at least one inch of water over the egg.

Place the saucepan on the stove and turn the heat on high.

Let the egg boil for 20 to 25 minutes.

Remove the saucepan from the fire and drain the water out carefully. Make sure you do not break the egg. It might help to detain the egg with a spoon to prevent it from falling into the sink.

Run cold water over the egg until it has cooled. This will allow you to touch it without being burned.

Place the alum and vinegar in a small bowl and stir with a spoon. Make sure the alum is completely dissolved into the vinegar.

Dip the paint brush into the solution and write a secret message on the egg's shell. It might be difficult to see what you are writing, so try to be as accurate as possible.

Let the solution permeate onto the inside of the egg by letting it dry. You can speed this process up by placing the egg under a bright light for 10 to 15 minutes.

Remove the shell from the egg and you will see your secret message written on the egg.