- Invisible Ink
- Those wishing to conceal the true contents of their missives from third parties often resort to penning their letters with invisible ink. A message set down in this manner becomes visible only when the document is treated with a substance that reveals the writing.
- Some use invisible ink as a dye, soaking entire pages in it instead of using it to write. The recipient of a message thus treated can confirm its authenticity by applying a substance that will turn the page a specific color, usually blue.
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