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You can use lemon juice, whole milk, or cream to write (or paint) an invisible message on a piece of white paper.
To reveal the message you need to heat the paper up. Get a grownup to help you by putting the paper in a hot frying pan and hold it down with a pot holder until the secret message appears.
Another method for revealing the secret message is to have an adult hold the paper over a hot incandescent light bulb and wait for the message to appear.