Invisible inks are special substances that can be used to write messages that are not visible to the naked eye. These inks often become visible when exposed to certain conditions, such as heat, ultraviolet light, or specific chemicals. They have been used throughout history for secret communication, especially during wartime. There are various types of invisible inks, including those made from lemon juice, milk, or vinegar. When heated, these substances can darken and reveal the hidden message. Today, invisible inks are also used in security features for banknotes and identification cards to prevent counterfeiting.