Invisible ink 

The simplest forms of invisible ink are lemon juice, and milk. For this type of 'heat fixed' ink, any acidic fluid will work. Write on paper with a fountain pen, toothpick[?] or a finger dipped in the liquid. Once dry, the paper appears blank. The writing is made to appear by heating the paper, on a radiator, iron or oven[?] for example.

Other types of invisble ink use different chemical reactions, usually an acid-base reaction (like litmus paper) this is similar to the blueprint process.

These dual chemical ink/decoder pairs would use a spray bottle for the decoding fluid, or vapor (ammonia fumes decode something, I forget what, maybe lemon juice.)

Invisible ink pens often have two tips, one the encoding tip, and one the decoding. (CMB I believe in most cases one tip would be used for the invisible message, the other for the 'cover' message. An invisible message sent as a blank sheet of paper screams to be decoded.)

Invisible ink is sometimes used to print parts of pictures or text in books for children to play with, particularly while they are travelling; a decoding pen is included with these books so that the children may rub the decoding pen over the invisible part of the text or picture, revealing the answer to a question printed in regular ink, the missing part of a picture, or the like.

Very rarely, invisible ink has been used in art. It is usually decoded, though when it isn't, it makes a mockery of the concept of "visual art".

suggestions for more on this topic

• how it works chemically
• was invisible ink ever used for actual espionage (yes: see Kahn's The Codebreakers)

See also: cryptography

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