Invisible ink is a writing fluid that is invisible under normal conditions but can be revealed by applying heat, light, or a chemical reagent. It has been used for centuries for secret communication, and it is still used today for a variety of purposes, including security, espionage, and counterfeiting.
There are many different recipes for invisible ink, but the most common ingredients include lemon juice, vinegar, and milk. These liquids are invisible when they are applied to paper, but they will turn brown when they are heated. Another popular recipe for invisible ink uses a solution of copper sulfate and water. This solution is invisible when it is applied to paper, but it will turn blue when it is exposed to light.
