Secret writing is very old and there are lots of invisible ink recipes. These inks work in different ways to hide and reveal messages written with them.
Some invisible inks—like milk, vinegar, or lemon juice—darken when they're heated. You can read messages written with them because they burn faster than the paper they're written on.
Other secret inks are solutions of chemicals that are colorless when dry but become visible when treated with another chemical, called a reagent.
Messages can even be written inside eggs, using a mixture of vinegar and a chemical called alum. Messages written with this ink are absorbed through the shell. When the egg is boiled and peeled, the message appears.
Spies not only need to hide their messages, they need to hide their secret inks. One method they use is to soak their shoelaces in the chemicals, then dry them in the sun. To make secret ink, they just put the shoelaces back in water.
In an emergency, spies can even use urine or a mixture of spit and pigeon poop as an invisible ink. But…would you want to receive that letter?