Existing comment:
Secret Writing, Flaps and Seals:
Sometimes a written communication can be hidden in plain view.
Secret writing insures that only the intended recipient will find and translate the message. Writings can be in code, or can even be invisible. The "wet system" uses invisible inks that appear when exposed to heat or chemical reactants. The "transfer" system simply involves carbon paper.
"Flaps and seals" work in the secret opening of mail -- by steaming open an envelope, separating the glue or using water to break the seal. |