Invisible ink is a special type of ink that is not visible to the naked eye when applied to paper or other surfaces. It can be made from various substances, such as lemon juice or vinegar, which become visible when heated or treated with certain chemicals. This technique has been used for centuries, especially in espionage and secret communications. Historically, invisible ink was popular during times of war, such as the American Revolutionary War, where it allowed spies to send messages without detection. Today, it is often used in security printing and forensic science to protect sensitive information and verify authenticity.