Invisible ink is a writing fluid that is not visible when applied to paper but can be made visible by heat, light, or chemical treatment. It has been used for centuries to send secret messages.
One of the most common ways to make invisible ink is to use lemon juice. Simply write your message on paper with a lemon wedge, and allow it to dry completely. The message will be invisible until it is heated, which will cause the lemon juice to turn brown and the message to become visible.