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To make an invisible ink message that will be revealed by heat, first pour 1 teaspoon (5 mL) of lemon juice into a small bowl. Add 1/2 teaspoon (2.5 mL) of water and mix it with a spoon. Take a sheet of white paper and place it on top of a towel. To write your message, take a cotton swab and dip it into the lemon juice and water. Write with the tip of your cotton swab, dipping the cotton swab into the lemon juice after every 2-3 letters to give it fresh ink. Once you’ve written your message, wait 2-3 minutes to give your paper time to dry. For the message to be revealed, hold the paper up to a hot lightbulb or leave it on a radiator to heat the paper. As the heat burns off the lemon juice, the message will be revealed! To learn how to make invisible ink with other common household ingredients, keep reading!