Invisible ink is a special type of ink that is not visible to the naked eye when it dries. It can be made from various substances, such as lemon juice or vinegar, which become clear when applied to paper. To reveal the hidden message, heat is often applied, causing the ink to change color and become visible. Historically, invisible ink has been used for secret communication, especially during wartime. Spies and intelligence agents would write messages that could only be seen by the intended recipient, ensuring that sensitive information remained confidential. Today, it is also used in various security applications, such as on banknotes and official documents.