Milk ink
Another common food stuff we all know and like. Milk is always present in our life and we use it for number purposes. We add milk to our coffee as we like to have it with milk, sure mostly people have their coffee with creams, but creams used to be a milk, and so it is a milk in some way. It is milk the only food we eat at those first days of our life.
Doctors say that milk is very useful for children and so advice parents to give milk their children in order they were string and healthy. And children also like milk. They are crazy about their breakfasts of milk and some kind of cereal. Though, they don't know that they can also use milk for another, more interesting purpose.
Milk is a natural invisible ink one may use for writing his secret letters. But it is wrong to recognize the milk ink just like a kid-toy. According to history, there were a great number of leaders of different countries who used milk ink as their ink for writing secret messages. There was a Russian leader who wrote his letters with milk when he was imprisoned.
So, it won't take many efforts for one to use milk as an invisible ink for secret letter. It is enough to have milk and some stuff to write with. After the paper gets dry there will be not a single mark on it. Though, it will be enough for recipient to work up the paper with thermal influence. For this purpose one may use iron or other heater and letter of message will grow brown.
